peewee.py

# May you do good and not evil
# May you find forgiveness for yourself and forgive others
# May you share freely, never taking more than you give.  -- SQLite source code
#
# As we enjoy great advantages from the inventions of others, we should be glad
# of an opportunity to serve others by an invention of ours, and this we should
# do freely and generously.  -- Ben Franklin
#
#     (\
#     (  \  /(o)\     caw!
#     (   \/  ()/ /)
#      (   `;.))'".)
#       `(/////.-'
#    =====))=))===()
#      ///'
#     //
#    '

import datetime
import decimal
import hashlib
import logging
import operator
import re
import sys
import threading
import uuid
from collections import deque
from collections import namedtuple
from copy import deepcopy
from functools import wraps
from inspect import isclass

__version__ = '2.3.0'
__all__ = [
    'BareField',
    'BigIntegerField',
    'BlobField',
    'BooleanField',
    'CharField',
    'Check',
    'Clause',
    'CompositeKey',
    'DatabaseError',
    'DataError',
    'DateField',
    'DateTimeField',
    'DecimalField',
    'DoesNotExist',
    'DoubleField',
    'DQ',
    'Field',
    'FloatField',
    'fn',
    'ForeignKeyField',
    'ImproperlyConfigured',
    'IntegerField',
    'IntegrityError',
    'InterfaceError',
    'InternalError',
    'JOIN_FULL',
    'JOIN_INNER',
    'JOIN_LEFT_OUTER',
    'Model',
    'MySQLDatabase',
    'NotSupportedError',
    'OperationalError',
    'Param',
    'PostgresqlDatabase',
    'prefetch',
    'PrimaryKeyField',
    'ProgrammingError',
    'Proxy',
    'R',
    'SqliteDatabase',
    'SQL',
    'TextField',
    'TimeField',
    'UUIDField',
    'Window',
]

# Set default logging handler to avoid "No handlers could be found for logger
# "peewee"" warnings.
try:  # Python 2.7+
    from logging import NullHandler
except ImportError:
    class NullHandler(logging.Handler):
        def emit(self, record):
            pass

# All peewee-generated logs are logged to this namespace.
logger = logging.getLogger('peewee')
logger.addHandler(NullHandler())

# Python 2/3 compatibility helpers. These helpers are used internally and are
# not exported.
def with_metaclass(meta, base=object):
    return meta("NewBase", (base,), {})

PY2 = sys.version_info[0] == 2
PY3 = sys.version_info[0] == 3
if PY3:
    import builtins
    from collections import Callable
    from functools import reduce
    callable = lambda c: isinstance(c, Callable)
    unicode_type = str
    string_type = bytes
    basestring = str
    print_ = getattr(builtins, 'print')
    binary_construct = lambda s: bytes(s.encode('raw_unicode_escape'))
    def reraise(tp, value, tb=None):
        if value.__traceback__ is not tb:
            raise value.with_traceback(tb)
        raise value
elif PY2:
    unicode_type = unicode
    string_type = basestring
    binary_construct = buffer
    def print_(s):
        sys.stdout.write(s)
        sys.stdout.write('\n')
    exec('def reraise(tp, value, tb=None): raise tp, value, tb')
else:
    raise RuntimeError('Unsupported python version.')

# By default, peewee supports Sqlite, MySQL and Postgresql.
try:
    import sqlite3
except ImportError:
    try:
        from pysqlite2 import dbapi2 as sqlite3
    except ImportError:
        sqlite3 = None
try:
    import psycopg2
    from psycopg2 import extensions as pg_extensions
except ImportError:
    psycopg2 = None
try:
    import MySQLdb as mysql  # prefer the C module.
except ImportError:
    try:
        import pymysql as mysql
    except ImportError:
        mysql = None

if sqlite3:
    sqlite3.register_adapter(decimal.Decimal, str)
    sqlite3.register_adapter(datetime.date, str)
    sqlite3.register_adapter(datetime.time, str)

DATETIME_PARTS = ['year', 'month', 'day', 'hour', 'minute', 'second']
DATETIME_LOOKUPS = set(DATETIME_PARTS)

# Sqlite does not support the `date_part` SQL function, so we will define an
# implementation in python.
SQLITE_DATETIME_FORMATS = (
    '%Y-%m-%d %H:%M:%S',
    '%Y-%m-%d %H:%M:%S.%f',
    '%Y-%m-%d',
    '%H:%M:%S',
    '%H:%M:%S.%f',
    '%H:%M')

def _sqlite_date_part(lookup_type, datetime_string):
    assert lookup_type in DATETIME_LOOKUPS
    dt = format_date_time(datetime_string, SQLITE_DATETIME_FORMATS)
    return getattr(dt, lookup_type)

SQLITE_DATE_TRUNC_MAPPING = {
    'year': '%Y',
    'month': '%Y-%m',
    'day': '%Y-%m-%d',
    'hour': '%Y-%m-%d %H',
    'minute': '%Y-%m-%d %H:%M',
    'second': '%Y-%m-%d %H:%M:%S'}
MYSQL_DATE_TRUNC_MAPPING = SQLITE_DATE_TRUNC_MAPPING.copy()
MYSQL_DATE_TRUNC_MAPPING['minute'] = '%Y-%m-%d %H:%i'
MYSQL_DATE_TRUNC_MAPPING['second'] = '%Y-%m-%d %H:%i:%S'

def _sqlite_date_trunc(lookup_type, datetime_string):
    assert lookup_type in SQLITE_DATE_TRUNC_MAPPING
    dt = format_date_time(datetime_string, SQLITE_DATETIME_FORMATS)
    return dt.strftime(SQLITE_DATE_TRUNC_MAPPING[lookup_type])

def _sqlite_regexp(regex, value):
    return re.search(regex, value, re.I) is not None

# Operators used in binary expressions.
OP_AND = 'and'
OP_OR = 'or'

OP_ADD = '+'
OP_SUB = '-'
OP_MUL = '*'
OP_DIV = '/'
OP_BIN_AND = '&'
OP_BIN_OR = '|'
OP_XOR = '^'
OP_MOD = '%'

OP_EQ = '='
OP_LT = '<'
OP_LTE = '<='
OP_GT = '>'
OP_GTE = '>='
OP_NE = '!='
OP_IN = 'in'
OP_IS = 'is'
OP_LIKE = 'like'
OP_ILIKE = 'ilike'
OP_BETWEEN = 'between'
OP_REGEXP = 'regexp'
OP_CONCAT = '||'

# To support "django-style" double-underscore filters, create a mapping between
# operation name and operation code, e.g. "__eq" == OP_EQ.
DJANGO_MAP = {
    'eq': OP_EQ,
    'lt': OP_LT,
    'lte': OP_LTE,
    'gt': OP_GT,
    'gte': OP_GTE,
    'ne': OP_NE,
    'in': OP_IN,
    'is': OP_IS,
    'like': OP_LIKE,
    'ilike': OP_ILIKE,
    'regexp': OP_REGEXP,
}

JOIN_INNER = 'inner'
JOIN_LEFT_OUTER = 'left outer'
JOIN_RIGHT_OUTER = 'right outer'
JOIN_FULL = 'full'

# Helper functions that are used in various parts of the codebase.
def merge_dict(source, overrides):
    merged = source.copy()
    merged.update(overrides)
    return merged

def returns_clone(func):
    """
    Method decorator that will "clone" the object before applying the given
    method.  This ensures that state is mutated in a more predictable fashion,
    and promotes the use of method-chaining.
    """
    def inner(self, *args, **kwargs):
        clone = self.clone()  # Assumes object implements `clone`.
        func(clone, *args, **kwargs)
        return clone
    inner.call_local = func  # Provide a way to call without cloning.
    return inner

def not_allowed(func):
    """
    Method decorator to indicate a method is not allowed to be called.  Will
    raise a `NotImplementedError`.
    """
    def inner(self, *args, **kwargs):
        raise NotImplementedError('%s is not allowed on %s instances' % (
            func, type(self).__name__))
    return inner

class Proxy(object):
    """
    Proxy class useful for situations when you wish to defer the initialization
    of an object.
    """
    __slots__ = ['obj', '_callbacks']

    def __init__(self):
        self._callbacks = []
        self.initialize(None)

    def initialize(self, obj):
        self.obj = obj
        for callback in self._callbacks:
            callback(obj)

    def attach_callback(self, callback):
        self._callbacks.append(callback)

    def __getattr__(self, attr):
        if self.obj is None:
            raise AttributeError('Cannot use uninitialized Proxy.')
        return getattr(self.obj, attr)

    def __setattr__(self, attr, value):
        if attr not in self.__slots__:
            raise AttributeError('Cannot set attribute on proxy.')
        return super(Proxy, self).__setattr__(attr, value)

class _CDescriptor(object):
    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return Entity(instance._alias)
        return self

# Classes representing the query tree.

class Node(object):
    """Base-class for any part of a query which shall be composable."""
    c = _CDescriptor()
    _node_type = 'node'

    def __init__(self):
        self._negated = False
        self._alias = None
        self._ordering = None  # ASC or DESC.

    def clone_base(self):
        return type(self)()

    def clone(self):
        inst = self.clone_base()
        inst._negated = self._negated
        inst._alias = self._alias
        inst._ordering = self._ordering
        return inst

    @returns_clone
    def __invert__(self):
        self._negated = not self._negated

    @returns_clone
    def alias(self, a=None):
        self._alias = a

    @returns_clone
    def asc(self):
        self._ordering = 'ASC'

    @returns_clone
    def desc(self):
        self._ordering = 'DESC'

    def _e(op, inv=False):
        """
        Lightweight factory which returns a method that builds an Expression
        consisting of the left-hand and right-hand operands, using `op`.
        """
        def inner(self, rhs):
            if inv:
                return Expression(rhs, op, self)
            return Expression(self, op, rhs)
        return inner
    __and__ = _e(OP_AND)
    __or__ = _e(OP_OR)

    __add__ = _e(OP_ADD)
    __sub__ = _e(OP_SUB)
    __mul__ = _e(OP_MUL)
    __div__ = _e(OP_DIV)
    __xor__ = _e(OP_XOR)
    __radd__ = _e(OP_ADD, inv=True)
    __rsub__ = _e(OP_SUB, inv=True)
    __rmul__ = _e(OP_MUL, inv=True)
    __rdiv__ = _e(OP_DIV, inv=True)
    __rand__ = _e(OP_AND, inv=True)
    __ror__ = _e(OP_OR, inv=True)
    __rxor__ = _e(OP_XOR, inv=True)

    def __eq__(self, rhs):
        if rhs is None:
            return Expression(self, OP_IS, None)
        return Expression(self, OP_EQ, rhs)
    def __ne__(self, rhs):
        if rhs is None:
            return ~Expression(self, OP_IS, None)
        return Expression(self, OP_NE, rhs)

    __lt__ = _e(OP_LT)
    __le__ = _e(OP_LTE)
    __gt__ = _e(OP_GT)
    __ge__ = _e(OP_GTE)
    __lshift__ = _e(OP_IN)
    __rshift__ = _e(OP_IS)
    __mod__ = _e(OP_LIKE)
    __pow__ = _e(OP_ILIKE)

    bin_and = _e(OP_BIN_AND)
    bin_or = _e(OP_BIN_OR)

    # Special expressions.
    def in_(self, *rhs):
        return Expression(self, OP_IN, rhs)
    def contains(self, rhs):
        return Expression(self, OP_ILIKE, '%%%s%%' % rhs)
    def startswith(self, rhs):
        return Expression(self, OP_ILIKE, '%s%%' % rhs)
    def endswith(self, rhs):
        return Expression(self, OP_ILIKE, '%%%s' % rhs)
    def between(self, low, high):
        return Expression(self, OP_BETWEEN, Clause(low, R('AND'), high))
    def regexp(self, expression):
        return Expression(self, OP_REGEXP, expression)
    def concat(self, rhs):
        return Expression(self, OP_CONCAT, rhs)

class Expression(Node):
    """A binary expression, e.g `foo + 1` or `bar < 7`."""
    _node_type = 'expression'

    def __init__(self, lhs, op, rhs, flat=False):
        super(Expression, self).__init__()
        self.lhs = lhs
        self.op = op
        self.rhs = rhs
        self.flat = flat

    def clone_base(self):
        return Expression(self.lhs, self.op, self.rhs, self.flat)

class DQ(Node):
    """A "django-style" filter expression, e.g. {'foo__eq': 'x'}."""
    def __init__(self, **query):
        super(DQ, self).__init__()
        self.query = query

    def clone_base(self):
        return DQ(**self.query)

class Param(Node):
    """
    Arbitrary parameter passed into a query. Instructs the query compiler to
    specifically treat this value as a parameter, useful for `list` which is
    special-cased for `IN` lookups.
    """
    _node_type = 'param'

    def __init__(self, value, conv=None):
        self.value = value
        self.conv = conv
        super(Param, self).__init__()

    def clone_base(self):
        return Param(self.value, self.conv)

class SQL(Node):
    """An unescaped SQL string, with optional parameters."""
    _node_type = 'sql'

    def __init__(self, value, *params):
        self.value = value
        self.params = params
        super(SQL, self).__init__()

    def clone_base(self):
        return SQL(self.value, *self.params)
R = SQL  # backwards-compat.

class Func(Node):
    """An arbitrary SQL function call."""
    _node_type = 'func'

    def __init__(self, name, *arguments):
        self.name = name
        self.arguments = arguments
        self._coerce = True
        super(Func, self).__init__()

    @returns_clone
    def coerce(self, coerce=True):
        self._coerce = coerce

    def clone_base(self):
        res = Func(self.name, *self.arguments)
        res._coerce = self._coerce
        return res

    def over(self, partition_by=None, order_by=None, window=None):
        if isinstance(partition_by, Window) and window is None:
            window = partition_by
        if window is None:
            sql = Window(
                partition_by=partition_by, order_by=order_by).__sql__()
        else:
            sql = SQL(window._alias)
        return Clause(self, SQL('OVER'), sql)

    def __getattr__(self, attr):
        def dec(*args, **kwargs):
            return Func(attr, *args, **kwargs)
        return dec

# fn is a factory for creating `Func` objects and supports a more friendly
# API.  So instead of `Func("LOWER", param)`, `fn.LOWER(param)`.
fn = Func(None)

class Window(Node):
    def __init__(self, partition_by=None, order_by=None):
        super(Window, self).__init__()
        self.partition_by = partition_by
        self.order_by = order_by
        self._alias = self._alias or 'w'

    def __sql__(self):
        over_clauses = []
        if self.partition_by:
            over_clauses.append(Clause(
                SQL('PARTITION BY'),
                CommaClause(*self.partition_by)))
        if self.order_by:
            over_clauses.append(Clause(
                SQL('ORDER BY'),
                CommaClause(*self.order_by)))
        return EnclosedClause(Clause(*over_clauses))

    def clone_base(self):
        return Window(self.partition_by, self.order_by)

class Clause(Node):
    """A SQL clause, one or more Node objects joined by spaces."""
    _node_type = 'clause'

    glue = ' '
    parens = False

    def __init__(self, *nodes):
        super(Clause, self).__init__()
        self.nodes = list(nodes)

    def clone_base(self):
        clone = Clause(*self.nodes)
        clone.glue = self.glue
        clone.parens = self.parens
        return clone

class CommaClause(Clause):
    """One or more Node objects joined by commas, no parens."""
    glue = ', '

class EnclosedClause(CommaClause):
    """One or more Node objects joined by commas and enclosed in parens."""
    parens = True

class Entity(Node):
    """A quoted-name or entity, e.g. "table"."column"."""
    _node_type = 'entity'

    def __init__(self, *path):
        super(Entity, self).__init__()
        self.path = path

    def clone_base(self):
        return Entity(*self.path)

    def __getattr__(self, attr):
        return Entity(*filter(None, self.path + (attr,)))

class Check(SQL):
    """Check constraint, usage: `Check('price > 10')`."""
    def __init__(self, value):
        super(Check, self).__init__('CHECK (%s)' % value)

class _StripParens(Node):
    _node_type = 'strip_parens'

    def __init__(self, node):
        super(_StripParens, self).__init__()
        self.node = node

Join = namedtuple('Join', ('dest', 'join_type', 'on'))

class FieldDescriptor(object):
    # Fields are exposed as descriptors in order to control access to the
    # underlying "raw" data.
    def __init__(self, field):
        self.field = field
        self.att_name = self.field.name

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return instance._data.get(self.att_name)
        return self.field

    def __set__(self, instance, value):
        instance._data[self.att_name] = value
        instance._dirty.add(self.att_name)

class Field(Node):
    """A column on a table."""
    _field_counter = 0
    _order = 0
    _node_type = 'field'
    db_field = 'unknown'

    def __init__(self, null=False, index=False, unique=False,
                 verbose_name=None, help_text=None, db_column=None,
                 default=None, choices=None, primary_key=False, sequence=None,
                 constraints=None, schema=None):
        self.null = null
        self.index = index
        self.unique = unique
        self.verbose_name = verbose_name
        self.help_text = help_text
        self.db_column = db_column
        self.default = default
        self.choices = choices  # Used for metadata purposes, not enforced.
        self.primary_key = primary_key
        self.sequence = sequence  # Name of sequence, e.g. foo_id_seq.
        self.constraints = constraints  # List of column constraints.
        self.schema = schema  # Name of schema, e.g. 'public'.

        # Used internally for recovering the order in which Fields were defined
        # on the Model class.
        Field._field_counter += 1
        self._order = Field._field_counter
        self._sort_key = (self.primary_key and 1 or 2), self._order

        self._is_bound = False  # Whether the Field is "bound" to a Model.
        super(Field, self).__init__()

    def clone_base(self, **kwargs):
        inst = type(self)(
            null=self.null,
            index=self.index,
            unique=self.unique,
            verbose_name=self.verbose_name,
            help_text=self.help_text,
            db_column=self.db_column,
            default=self.default,
            choices=self.choices,
            primary_key=self.primary_key,
            sequence=self.sequence,
            constraints=self.constraints,
            schema=self.schema,
            **kwargs)
        if self._is_bound:
            inst.name = self.name
            inst.model_class = self.model_class
            return inst

    def add_to_class(self, model_class, name):
        """
        Hook that replaces the `Field` attribute on a class with a named
        `FieldDescriptor`. Called by the metaclass during construction of the
        `Model`.
        """
        self.name = name
        self.model_class = model_class
        self.db_column = self.db_column or self.name
        if not self.verbose_name:
            self.verbose_name = re.sub('_+', ' ', name).title()

        model_class._meta.fields[self.name] = self
        model_class._meta.columns[self.db_column] = self

        setattr(model_class, name, FieldDescriptor(self))
        self._is_bound = True

    def get_database(self):
        return self.model_class._meta.database

    def get_column_type(self):
        field_type = self.get_db_field()
        return self.get_database().compiler().get_column_type(field_type)

    def get_db_field(self):
        return self.db_field

    def get_modifiers(self):
        return None

    def coerce(self, value):
        return value

    def db_value(self, value):
        """Convert the python value for storage in the database."""
        return value if value is None else self.coerce(value)

    def python_value(self, value):
        """Convert the database value to a pythonic value."""
        return value if value is None else self.coerce(value)

    def _as_entity(self, with_table=False):
        if with_table:
            return Entity(self.model_class._meta.db_table, self.db_column)
        return Entity(self.db_column)

    def __ddl_column__(self, column_type):
        """Return the column type, e.g. VARCHAR(255) or REAL."""
        modifiers = self.get_modifiers()
        if modifiers:
            return SQL(
                '%s(%s)' % (column_type, ', '.join(map(str, modifiers))))
        return SQL(column_type)

    def __ddl__(self, column_type):
        """Return a list of Node instances that defines the column."""
        ddl = [self._as_entity(), self.__ddl_column__(column_type)]
        if not self.null:
            ddl.append(SQL('NOT NULL'))
        if self.primary_key:
            ddl.append(SQL('PRIMARY KEY'))
        if self.sequence:
            ddl.append(SQL("DEFAULT NEXTVAL('%s')" % self.sequence))
        if self.constraints:
            ddl.extend(self.constraints)
        return ddl

    def __hash__(self):
        return hash(self.name + '.' + self.model_class.__name__)

class BareField(Field):
    db_field = 'bare'

class IntegerField(Field):
    db_field = 'int'
    coerce = int

class BigIntegerField(IntegerField):
    db_field = 'bigint'

class PrimaryKeyField(IntegerField):
    db_field = 'primary_key'

    def __init__(self, *args, **kwargs):
        kwargs['primary_key'] = True
        super(PrimaryKeyField, self).__init__(*args, **kwargs)

class FloatField(Field):
    db_field = 'float'
    coerce = float

class DoubleField(FloatField):
    db_field = 'double'

class DecimalField(Field):
    db_field = 'decimal'

    def __init__(self, max_digits=10, decimal_places=5, auto_round=False,
                 rounding=None, *args, **kwargs):
        self.max_digits = max_digits
        self.decimal_places = decimal_places
        self.auto_round = auto_round
        self.rounding = rounding or decimal.DefaultContext.rounding
        super(DecimalField, self).__init__(*args, **kwargs)

    def clone_base(self, **kwargs):
        return super(DecimalField, self).clone_base(
            max_digits=self.max_digits,
            decimal_places=self.decimal_places,
            auto_round=self.auto_round,
            rounding=self.rounding,
            **kwargs)

    def get_modifiers(self):
        return [self.max_digits, self.decimal_places]

    def db_value(self, value):
        D = decimal.Decimal
        if not value:
            return value if value is None else D(0)
        if self.auto_round:
            exp = D(10) ** (-self.decimal_places)
            rounding = self.rounding
            return D(str(value)).quantize(exp, rounding=rounding)
        return value

    def python_value(self, value):
        if value is not None:
            if isinstance(value, decimal.Decimal):
                return value
            return decimal.Decimal(str(value))

def coerce_to_unicode(s, encoding='utf-8'):
    if isinstance(s, unicode_type):
        return s
    elif isinstance(s, string_type):
        return s.decode(encoding)
    return unicode_type(s)

class CharField(Field):
    db_field = 'string'

    def __init__(self, max_length=255, *args, **kwargs):
        self.max_length = max_length
        super(CharField, self).__init__(*args, **kwargs)

    def clone_base(self, **kwargs):
        return super(CharField, self).clone_base(
            max_length=self.max_length,
            **kwargs)

    def get_modifiers(self):
        return self.max_length and [self.max_length] or None

    def coerce(self, value):
        return coerce_to_unicode(value or '')

class TextField(Field):
    db_field = 'text'

    def coerce(self, value):
        return coerce_to_unicode(value or '')

class BlobField(Field):
    db_field = 'blob'

    def db_value(self, value):
        if isinstance(value, basestring):
            return binary_construct(value)
        return value

class UUIDField(Field):
    db_field = 'uuid'

    def db_value(self, value):
        return None if value is None else str(value)

    def python_value(self, value):
        return None if value is None else uuid.UUID(value)

def format_date_time(value, formats, post_process=None):
    post_process = post_process or (lambda x: x)
    for fmt in formats:
        try:
            return post_process(datetime.datetime.strptime(value, fmt))
        except ValueError:
            pass
    return value

def _date_part(date_part):
    def dec(self):
        return self.model_class._meta.database.extract_date(date_part, self)
    return dec

class _BaseFormattedField(Field):
    formats = None
    def __init__(self, formats=None, *args, **kwargs):
        if formats is not None:
            self.formats = formats
        super(_BaseFormattedField, self).__init__(*args, **kwargs)

    def clone_base(self, **kwargs):
        return super(_BaseFormattedField, self).clone_base(
            formats=self.formats,
            **kwargs)

class DateTimeField(_BaseFormattedField):
    db_field = 'datetime'
    formats = [
        '%Y-%m-%d %H:%M:%S.%f',
        '%Y-%m-%d %H:%M:%S',
        '%Y-%m-%d',
    ]

    def python_value(self, value):
        if value and isinstance(value, basestring):
            return format_date_time(value, self.formats)
        return value

    year = property(_date_part('year'))
    month = property(_date_part('month'))
    day = property(_date_part('day'))
    hour = property(_date_part('hour'))
    minute = property(_date_part('minute'))
    second = property(_date_part('second'))

class DateField(_BaseFormattedField):
    db_field = 'date'
    formats = [
        '%Y-%m-%d',
        '%Y-%m-%d %H:%M:%S',
        '%Y-%m-%d %H:%M:%S.%f',
    ]

    def python_value(self, value):
        if value and isinstance(value, basestring):
            pp = lambda x: x.date()
            return format_date_time(value, self.formats, pp)
        elif value and isinstance(value, datetime.datetime):
            return value.date()
        return value

    year = property(_date_part('year'))
    month = property(_date_part('month'))
    day = property(_date_part('day'))

class TimeField(_BaseFormattedField):
    db_field = 'time'
    formats = [
        '%H:%M:%S.%f',
        '%H:%M:%S',
        '%H:%M',
        '%Y-%m-%d %H:%M:%S.%f',
        '%Y-%m-%d %H:%M:%S',
    ]

    def python_value(self, value):
        if value and isinstance(value, basestring):
            pp = lambda x: x.time()
            return format_date_time(value, self.formats, pp)
        elif value and isinstance(value, datetime.datetime):
            return value.time()
        return value

    hour = property(_date_part('hour'))
    minute = property(_date_part('minute'))
    second = property(_date_part('second'))

class BooleanField(Field):
    db_field = 'bool'
    coerce = bool

class RelationDescriptor(FieldDescriptor):
    """Foreign-key abstraction to replace a related PK with a related model."""
    def __init__(self, field, rel_model):
        self.rel_model = rel_model
        super(RelationDescriptor, self).__init__(field)

    def get_object_or_id(self, instance):
        rel_id = instance._data.get(self.att_name)
        if rel_id is not None or self.att_name in instance._obj_cache:
            if self.att_name not in instance._obj_cache:
                obj = self.rel_model.get(self.field.to_field == rel_id)
                instance._obj_cache[self.att_name] = obj
            return instance._obj_cache[self.att_name]
        elif not self.field.null:
            raise self.rel_model.DoesNotExist
        return rel_id

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return self.get_object_or_id(instance)
        return self.field

    def __set__(self, instance, value):
        if isinstance(value, self.rel_model):
            instance._data[self.att_name] = getattr(
                value, self.field.to_field.name)
            instance._obj_cache[self.att_name] = value
        else:
            orig_value = instance._data.get(self.att_name)
            instance._data[self.att_name] = value
            if orig_value != value and self.att_name in instance._obj_cache:
                del instance._obj_cache[self.att_name]
        instance._dirty.add(self.att_name)

class ReverseRelationDescriptor(object):
    """Back-reference to expose related objects as a `SelectQuery`."""
    def __init__(self, field):
        self.field = field
        self.rel_model = field.model_class

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return self.rel_model.select().where(
                self.field == getattr(instance, self.field.to_field.name))
        return self

class ForeignKeyField(IntegerField):
    def __init__(self, rel_model, related_name=None, on_delete=None,
                 on_update=None, extra=None, to_field=None, *args, **kwargs):
        if rel_model != 'self' and not isinstance(rel_model, Proxy) and not \
                issubclass(rel_model, Model):
            raise TypeError('Unexpected value for `rel_model`.  Expected '
                            '`Model`, `Proxy` or "self"')
        self.rel_model = rel_model
        self._related_name = related_name
        self.deferred = isinstance(rel_model, Proxy)
        self.on_delete = on_delete
        self.on_update = on_update
        self.extra = extra
        self.to_field = to_field
        super(ForeignKeyField, self).__init__(*args, **kwargs)

    def clone_base(self, **kwargs):
        return super(ForeignKeyField, self).clone_base(
            rel_model=self.rel_model,
            related_name=self.related_name,
            on_delete=self.on_delete,
            on_update=self.on_update,
            extra=self.extra,
            to_field=self.to_field,
            **kwargs)

    def add_to_class(self, model_class, name):
        if isinstance(self.rel_model, Proxy):
            def callback(rel_model):
                self.rel_model = rel_model
                self.add_to_class(model_class, name)
            self.rel_model.attach_callback(callback)
            return

        self.name = name
        self.model_class = model_class
        self.db_column = self.db_column or '%s_id' % self.name
        if not self.verbose_name:
            self.verbose_name = re.sub('_+', ' ', name).title()

        model_class._meta.fields[self.name] = self
        model_class._meta.columns[self.db_column] = self

        model_name = model_class._meta.name
        self.related_name = self._related_name or '%s_set' % (model_name)

        if self.rel_model == 'self':
            self.rel_model = self.model_class

        if self.to_field is not None:
            if not isinstance(self.to_field, Field):
                self.to_field = getattr(self.rel_model, self.to_field)
        else:
            self.to_field = self.rel_model._meta.primary_key

        if self.related_name in self.rel_model._meta.fields:
            error = ('Foreign key: %s.%s related name "%s" collision with '
                     'model field of the same name.')
            params = self.model_class._meta.name, self.name, self.related_name
            raise AttributeError(error % params)
        if self.related_name in self.rel_model._meta.reverse_rel:
            error = ('Foreign key: %s.%s related name "%s" collision with '
                     'foreign key using same related_name.')
            params = self.model_class._meta.name, self.name, self.related_name
            raise AttributeError(error % params)

        fk_descriptor = RelationDescriptor(self, self.rel_model)
        backref_descriptor = ReverseRelationDescriptor(self)
        setattr(model_class, name, fk_descriptor)
        setattr(self.rel_model, self.related_name, backref_descriptor)
        self._is_bound = True

        model_class._meta.rel[self.name] = self
        self.rel_model._meta.reverse_rel[self.related_name] = self

    def get_db_field(self):
        """
        Overridden to ensure Foreign Keys use same column type as the primary
        key they point to.
        """
        if not isinstance(self.to_field, PrimaryKeyField):
            return self.to_field.get_db_field()
        return super(ForeignKeyField, self).get_db_field()

    def get_modifiers(self):
        if not isinstance(self.to_field, PrimaryKeyField):
            return self.to_field.get_modifiers()
        return super(ForeignKeyField, self).get_modifiers()

    def coerce(self, value):
        return self.to_field.coerce(value)

    def db_value(self, value):
        if isinstance(value, self.rel_model):
            value = value.get_id()
        return self.to_field.db_value(value)


class CompositeKey(object):
    """A primary key composed of multiple columns."""
    sequence = None

    def __init__(self, *field_names):
        self.field_names = field_names

    def add_to_class(self, model_class, name):
        self.name = name
        self.model_class = model_class
        setattr(model_class, name, self)

    def __get__(self, instance, instance_type=None):
        if instance is not None:
            return [getattr(instance, field_name)
                    for field_name in self.field_names]
        return self

    def __set__(self, instance, value):
        pass

    def __eq__(self, other):
        expressions = [(self.model_class._meta.fields[field] == value)
                       for field, value in zip(self.field_names, other)]
        return reduce(operator.and_, expressions)


class AliasMap(object):
    prefix = 't'

    def __init__(self):
        self._alias_map = {}
        self._counter = 0

    def __repr__(self):
        return '<AliasMap: %s>' % self._alias_map

    def add(self, obj, alias=None):
        if obj in self._alias_map:
            return
        self._counter += 1
        self._alias_map[obj] = alias or '%s%s' % (self.prefix, self._counter)

    def __getitem__(self, obj):
        if obj not in self._alias_map:
            self.add(obj)
        return self._alias_map[obj]

    def __contains__(self, obj):
        return obj in self._alias_map

    def update(self, alias_map):
        if alias_map:
            for obj, alias in alias_map._alias_map.items():
                if obj not in self:
                    self._alias_map[obj] = alias
        return self


class QueryCompiler(object):
    # Mapping of `db_type` to actual column type used by database driver.
    # Database classes may provide additional column types or overrides.
    field_map = {
        'bare': '',
        'bigint': 'BIGINT',
        'blob': 'BLOB',
        'bool': 'SMALLINT',
        'date': 'DATE',
        'datetime': 'DATETIME',
        'decimal': 'DECIMAL',
        'double': 'REAL',
        'float': 'REAL',
        'int': 'INTEGER',
        'primary_key': 'INTEGER',
        'string': 'VARCHAR',
        'text': 'TEXT',
        'time': 'TIME',
    }

    # Mapping of OP_ to actual SQL operation.  For most databases this will be
    # the same, but some column types or databases may support additional ops.
    # Like `field_map`, Database classes may extend or override these.
    op_map = {
        OP_EQ: '=',
        OP_LT: '<',
        OP_LTE: '<=',
        OP_GT: '>',
        OP_GTE: '>=',
        OP_NE: '!=',
        OP_IN: 'IN',
        OP_IS: 'IS',
        OP_BIN_AND: '&',
        OP_BIN_OR: '|',
        OP_LIKE: 'LIKE',
        OP_ILIKE: 'ILIKE',
        OP_BETWEEN: 'BETWEEN',
        OP_ADD: '+',
        OP_SUB: '-',
        OP_MUL: '*',
        OP_DIV: '/',
        OP_XOR: '#',
        OP_AND: 'AND',
        OP_OR: 'OR',
        OP_MOD: '%',
        OP_REGEXP: 'REGEXP',
        OP_CONCAT: '||',
    }

    join_map = {
        JOIN_INNER: 'INNER',
        JOIN_LEFT_OUTER: 'LEFT OUTER',
        JOIN_RIGHT_OUTER: 'RIGHT OUTER',
        JOIN_FULL: 'FULL',
    }
    alias_map_class = AliasMap

    def __init__(self, quote_char='"', interpolation='?', field_overrides=None,
                 op_overrides=None):
        self.quote_char = quote_char
        self.interpolation = interpolation
        self._field_map = merge_dict(self.field_map, field_overrides or {})
        self._op_map = merge_dict(self.op_map, op_overrides or {})
        self._parse_map = self.get_parse_map()
        self._unknown_types = set(['param'])

    def get_parse_map(self):
        return {
            'expression': self._parse_expression,
            'param': self._parse_param,
            'func': self._parse_func,
            'clause': self._parse_clause,
            'entity': self._parse_entity,
            'field': self._parse_field,
            'sql': self._parse_sql,
            'select_query': self._parse_select_query,
            'compound_select_query': self._parse_compound_select_query,
            'strip_parens': self._parse_strip_parens,
        }

    def quote(self, s):
        return '%s%s%s' % (self.quote_char, s, self.quote_char)

    def get_column_type(self, f):
        return self._field_map[f]

    def get_op(self, q):
        return self._op_map[q]

    def _sorted_fields(self, field_dict):
        return sorted(field_dict.items(), key=lambda i: i[0]._sort_key)

    def _clean_extra_parens(self, s):
        ct = i = 0
        l = len(s)
        while i < l:
            if s[i] == '(' and s[l - 1] == ')':
                ct += 1
                i += 1
                l -= 1
            else:
                break
        if ct:
            return s[ct:-ct]
        return s

    def _parse_default(self, node, alias_map, conv):
        return self.interpolation, [node]

    def _parse_expression(self, node, alias_map, conv):
        if isinstance(node.lhs, Field):
            conv = node.lhs
        lhs, lparams = self.parse_node(node.lhs, alias_map, conv)
        rhs, rparams = self.parse_node(node.rhs, alias_map, conv)
        template = '%s %s %s' if node.flat else '(%s %s %s)'
        sql = template % (lhs, self.get_op(node.op), rhs)
        return sql, lparams + rparams

    def _parse_param(self, node, alias_map, conv):
        if node.conv:
            params = [node.conv(node.value)]
        else:
            params = [node.value]
        return self.interpolation, params

    def _parse_func(self, node, alias_map, conv):
        conv = node._coerce and conv or None
        sql, params = self.parse_node_list(node.arguments, alias_map, conv)
        return '%s(%s)' % (node.name, self._clean_extra_parens(sql)), params

    def _parse_clause(self, node, alias_map, conv):
        sql, params = self.parse_node_list(
            node.nodes, alias_map, conv, node.glue)
        if node.parens:
            sql = '(%s)' % self._clean_extra_parens(sql)
        return sql, params

    def _parse_entity(self, node, alias_map, conv):
        return '.'.join(map(self.quote, node.path)), []

    def _parse_sql(self, node, alias_map, conv):
        return node.value, list(node.params)

    def _parse_field(self, node, alias_map, conv):
        if alias_map:
            sql = '.'.join((
                alias_map[node.model_class], self.quote(node.db_column)))
        else:
            sql = self.quote(node.db_column)
        return sql, []

    def _parse_compound_select_query(self, node, alias_map, conv):
        l, lp = self.generate_select(node.lhs, alias_map)
        r, rp = self.generate_select(node.rhs, alias_map)
        sql = '%s %s %s' % (l, node.operator, r)
        return sql, lp + rp

    def _parse_select_query(self, node, alias_map, conv):
        clone = node.clone()
        if not node._explicit_selection:
            if conv and isinstance(conv, ForeignKeyField):
                select_field = conv.to_field
            else:
                select_field = clone.model_class._meta.primary_key
            clone._select = (select_field,)
        sub, params = self.generate_select(clone, alias_map)
        return '(%s)' % self._clean_extra_parens(sub), params

    def _parse_strip_parens(self, node, alias_map, conv):
        sql, params = self.parse_node(node.node, alias_map, conv)
        return self._clean_extra_parens(sql), params

    def _parse(self, node, alias_map, conv):
        # By default treat the incoming node as a raw value that should be
        # parameterized.
        node_type = getattr(node, '_node_type', None)
        unknown = False
        if node_type in self._parse_map:
            sql, params = self._parse_map[node_type](node, alias_map, conv)
            unknown = node_type in self._unknown_types
        elif isinstance(node, (list, tuple)):
            # If you're wondering how to pass a list into your query, simply
            # wrap it in Param().
            sql, params = self.parse_node_list(node, alias_map, conv)
            sql = '(%s)' % sql
        elif isinstance(node, Model):
            sql = self.interpolation
            if conv and isinstance(conv, ForeignKeyField):
                params = [
                    conv.to_field.db_value(getattr(node, conv.to_field.name))]
            else:
                params = [node.get_id()]
        elif (isclass(node) and issubclass(node, Model)) or \
                isinstance(node, ModelAlias):
            entity = node._as_entity().alias(alias_map[node])
            sql, params = self.parse_node(entity, alias_map, conv)
        else:
            sql, params = self._parse_default(node, alias_map, conv)
            unknown = True

        return sql, params, unknown

    def parse_node(self, node, alias_map=None, conv=None):
        sql, params, unknown = self._parse(node, alias_map, conv)
        if unknown and conv and params:
            params = [conv.db_value(i) for i in params]

        if isinstance(node, Node):
            if node._negated:
                sql = 'NOT %s' % sql
            if node._alias:
                sql = ' '.join((sql, 'AS', node._alias))
            if node._ordering:
                sql = ' '.join((sql, node._ordering))
        return sql, params

    def parse_node_list(self, nodes, alias_map, conv=None, glue=', '):
        sql = []
        params = []
        for node in nodes:
            node_sql, node_params = self.parse_node(node, alias_map, conv)
            sql.append(node_sql)
            params.extend(node_params)
        return glue.join(sql), params

    def calculate_alias_map(self, query, alias_map=None):
        new_map = self.alias_map_class()
        if alias_map is not None:
            new_map._counter = alias_map._counter

        new_map.add(query.model_class, query.model_class._meta.table_alias)
        for src_model, joined_models in query._joins.items():
            new_map.add(src_model, src_model._meta.table_alias)
            for join_obj in joined_models:
                new_map.add(join_obj.dest, join_obj.dest._meta.table_alias)

        return new_map.update(alias_map)

    def build_query(self, clauses, alias_map=None):
        return self.parse_node(Clause(*clauses), alias_map)

    def generate_joins(self, joins, model_class, alias_map):
        clauses = []
        seen = set()
        q = [model_class]
        while q:
            curr = q.pop()
            if curr not in joins or curr in seen:
                continue
            seen.add(curr)
            for join in joins[curr]:
                src = curr
                dest = join.dest
                if isinstance(join.on, Expression):
                    # Clear any alias on the join expression.
                    constraint = join.on.clone().alias()
                else:
                    field = src._meta.rel_for_model(dest, join.on)
                    if field:
                        left_field = field
                        right_field = field.to_field
                    else:
                        field = dest._meta.rel_for_model(src, join.on)
                        left_field = field.to_field
                        right_field = field
                    constraint = (left_field == right_field)

                if isinstance(dest, Node):
                    # TODO: ensure alias?
                    dest_n = dest
                else:
                    q.append(dest)
                    dest_n = dest._as_entity().alias(alias_map[dest])

                join_type = self.join_map[join.join_type or JOIN_INNER]
                join_stmt = SQL('%s JOIN' % (join_type))
                clauses.append(
                    Clause(join_stmt, dest_n, SQL('ON'), constraint))

        return clauses

    def generate_select(self, query, alias_map=None):
        model = query.model_class
        db = model._meta.database

        alias_map = self.calculate_alias_map(query, alias_map)

        if isinstance(query, CompoundSelect):
            clauses = [query]
        else:
            if not query._distinct:
                clauses = [SQL('SELECT')]
            else:
                clauses = [SQL('SELECT DISTINCT')]
                if query._distinct not in (True, False):
                    clauses += [SQL('ON'), EnclosedClause(*query._distinct)]

            select_clause = Clause(*query._select)
            select_clause.glue = ', '

            clauses.extend((select_clause, SQL('FROM')))
            if query._from is None:
                clauses.append(model._as_entity().alias(alias_map[model]))
            else:
                clauses.append(CommaClause(*query._from))

        if query._windows is not None:
            clauses.append(SQL('WINDOW'))
            clauses.append(CommaClause(*[
                Clause(
                    SQL(window._alias),
                    SQL('AS'),
                    window.__sql__())
                for window in query._windows]))

        join_clauses = self.generate_joins(query._joins, model, alias_map)
        if join_clauses:
            clauses.extend(join_clauses)

        if query._where is not None:
            clauses.extend([SQL('WHERE'), query._where])

        if query._group_by:
            clauses.extend([SQL('GROUP BY'), CommaClause(*query._group_by)])

        if query._having:
            clauses.extend([SQL('HAVING'), query._having])

        if query._order_by:
            clauses.extend([SQL('ORDER BY'), CommaClause(*query._order_by)])

        if query._limit or (query._offset and db.limit_max):
            limit = query._limit or db.limit_max
            clauses.append(SQL('LIMIT %s' % limit))
        if query._offset:
            clauses.append(SQL('OFFSET %s' % query._offset))

        for_update, no_wait = query._for_update
        if for_update:
            stmt = 'FOR UPDATE NOWAIT' if no_wait else 'FOR UPDATE'
            clauses.append(SQL(stmt))

        return self.build_query(clauses, alias_map)

    def generate_update(self, query):
        model = query.model_class
        alias_map = self.alias_map_class()
        alias_map.add(model, model._meta.db_table)
        clauses = [SQL('UPDATE'), model._as_entity(), SQL('SET')]

        update = []
        for field, value in self._sorted_fields(query._update):
            if not isinstance(value, (Node, Model)):
                value = Param(value, conv=field.db_value)
            update.append(Expression(
                field._as_entity(with_table=False),
                OP_EQ,
                value,
                flat=True))
        clauses.append(CommaClause(*update))

        if query._where:
            clauses.extend([SQL('WHERE'), query._where])

        return self.build_query(clauses, alias_map)

    def _get_field_clause(self, fields):
        return EnclosedClause(*[
            field._as_entity(with_table=False) for field in fields])

    def generate_insert(self, query):
        model = query.model_class
        alias_map = self.alias_map_class()
        alias_map.add(model, model._meta.db_table)
        statement = query._upsert and 'INSERT OR REPLACE INTO' or 'INSERT INTO'
        clauses = [SQL(statement), model._as_entity()]

        if query._query is not None:
            if query._fields:
                clauses.append(self._get_field_clause(query._fields))
            clauses.append(_StripParens(query._query))

        elif query._rows is not None:
            fields, value_clauses = [], []
            have_fields = False

            for row_dict in query._iter_rows():
                if not have_fields:
                    fields = sorted(
                        row_dict.keys(), key=operator.attrgetter('_sort_key'))
                    have_fields = True

                values = []
                for field in fields:
                    value = row_dict[field]
                    if not isinstance(value, (Node, Model)):
                        value = Param(value, conv=field.db_value)
                    values.append(value)

                value_clauses.append(EnclosedClause(*values))

            if fields:
                clauses.extend([
                    self._get_field_clause(fields),
                    SQL('VALUES'),
                    CommaClause(*value_clauses)])

        return self.build_query(clauses, alias_map)

    def generate_delete(self, query):
        model = query.model_class
        clauses = [SQL('DELETE FROM'), model._as_entity()]
        if query._where:
            clauses.extend([SQL('WHERE'), query._where])
        return self.build_query(clauses)

    def field_definition(self, field):
        column_type = self.get_column_type(field.get_db_field())
        ddl = field.__ddl__(column_type)
        return Clause(*ddl)

    def foreign_key_constraint(self, field):
        ddl = [
            SQL('FOREIGN KEY'),
            EnclosedClause(field._as_entity()),
            SQL('REFERENCES'),
            field.rel_model._as_entity(),
            EnclosedClause(field.to_field._as_entity())]
        if field.on_delete:
            ddl.append(SQL('ON DELETE %s' % field.on_delete))
        if field.on_update:
            ddl.append(SQL('ON UPDATE %s' % field.on_update))
        return Clause(*ddl)

    def return_parsed_node(function_name):
        # TODO: treat all `generate_` functions as returning clauses, instead
        # of SQL/params.
        def inner(self, *args, **kwargs):
            fn = getattr(self, function_name)
            return self.parse_node(fn(*args, **kwargs))
        return inner

    def _create_foreign_key(self, model_class, field, constraint=None):
        constraint = constraint or 'fk_%s_%s_refs_%s' % (
            model_class._meta.db_table,
            field.db_column,
            field.rel_model._meta.db_table)
        fk_clause = self.foreign_key_constraint(field)
        return Clause(
            SQL('ALTER TABLE'),
            model_class._as_entity(),
            SQL('ADD CONSTRAINT'),
            Entity(constraint),
            *fk_clause.nodes)
    create_foreign_key = return_parsed_node('_create_foreign_key')

    def _create_table(self, model_class, safe=False):
        statement = 'CREATE TABLE IF NOT EXISTS' if safe else 'CREATE TABLE'
        meta = model_class._meta

        columns, constraints = [], []
        if isinstance(meta.primary_key, CompositeKey):
            pk_cols = [meta.fields[f]._as_entity()
                       for f in meta.primary_key.field_names]
            constraints.append(Clause(
                SQL('PRIMARY KEY'), EnclosedClause(*pk_cols)))
        for field in meta.get_fields():
            columns.append(self.field_definition(field))
            if isinstance(field, ForeignKeyField) and not field.deferred:
                constraints.append(self.foreign_key_constraint(field))

        return Clause(
            SQL(statement),
            model_class._as_entity(),
            EnclosedClause(*(columns + constraints)))
    create_table = return_parsed_node('_create_table')

    def _drop_table(self, model_class, fail_silently=False, cascade=False):
        statement = 'DROP TABLE IF EXISTS' if fail_silently else 'DROP TABLE'
        ddl = [SQL(statement), model_class._as_entity()]
        if cascade:
            ddl.append(SQL('CASCADE'))
        return Clause(*ddl)
    drop_table = return_parsed_node('_drop_table')

    def index_name(self, table, columns):
        index = '%s_%s' % (table, '_'.join(columns))
        if len(index) > 64:
            index_hash = hashlib.md5(index.encode('utf-8')).hexdigest()
            index = '%s_%s' % (table, index_hash)
        return index

    def _create_index(self, model_class, fields, unique, *extra):
        tbl_name = model_class._meta.db_table
        statement = 'CREATE UNIQUE INDEX' if unique else 'CREATE INDEX'
        index_name = self.index_name(tbl_name, [f.db_column for f in fields])
        return Clause(
            SQL(statement),
            Entity(index_name),
            SQL('ON'),
            model_class._as_entity(),
            EnclosedClause(*[field._as_entity() for field in fields]),
            *extra)
    create_index = return_parsed_node('_create_index')

    def _create_sequence(self, sequence_name):
        return Clause(SQL('CREATE SEQUENCE'), Entity(sequence_name))
    create_sequence = return_parsed_node('_create_sequence')

    def _drop_sequence(self, sequence_name):
        return Clause(SQL('DROP SEQUENCE'), Entity(sequence_name))
    drop_sequence = return_parsed_node('_drop_sequence')


class QueryResultWrapper(object):
    """
    Provides an iterator over the results of a raw Query, additionally doing
    two things:
    - converts rows from the database into python representations
    - ensures that multiple iterations do not result in multiple queries
    """
    def __init__(self, model, cursor, meta=None):
        self.model = model
        self.cursor = cursor

        self.__ct = 0
        self.__idx = 0

        self._result_cache = []
        self._populated = False
        self._initialized = False

        if meta is not None:
            self.column_meta, self.join_meta = meta
        else:
            self.column_meta = self.join_meta = None

    def __iter__(self):
        self.__idx = 0

        if not self._populated:
            return self
        else:
            return iter(self._result_cache)

    def process_row(self, row):
        return row

    def iterate(self):
        row = self.cursor.fetchone()
        if not row:
            self._populated = True
            raise StopIteration
        elif not self._initialized:
            self.initialize(self.cursor.description)
            self._initialized = True
        return self.process_row(row)

    def iterator(self):
        while True:
            yield self.iterate()

    def next(self):
        if self.__idx < self.__ct:
            inst = self._result_cache[self.__idx]
            self.__idx += 1
            return inst

        obj = self.iterate()
        self._result_cache.append(obj)
        self.__ct += 1
        self.__idx += 1
        return obj
    __next__ = next

    def fill_cache(self, n=None):
        n = n or float('Inf')
        if n < 0:
            raise ValueError('Negative values are not supported.')
        self.__idx = self.__ct
        while not self._populated and (n > self.__ct):
            try:
                self.next()
            except StopIteration:
                break

class ExtQueryResultWrapper(QueryResultWrapper):
    def initialize(self, description):
        model = self.model
        conv = []
        identity = lambda x: x
        for i in range(len(description)):
            func = identity
            column = description[i][0]
            found = False
            if self.column_meta is not None:
                try:
                    select_column = self.column_meta[i]
                except IndexError:
                    pass
                else:
                    if isinstance(select_column, Field):
                        func = select_column.python_value
                        column = select_column._alias or select_column.name
                        found = True
                    elif (isinstance(select_column, Func) and
                            isinstance(select_column.arguments[0], Field)):
                        if select_column._coerce:
                            # Special-case handling aggregations.
                            func = select_column.arguments[0].python_value
                        found = True

            if not found and column in model._meta.columns:
                field_obj = model._meta.columns[column]
                column = field_obj.name
                func = field_obj.python_value

            conv.append((i, column, func))
        self.conv = conv

class TuplesQueryResultWrapper(ExtQueryResultWrapper):
    def process_row(self, row):
        return tuple([self.conv[i][2](col) for i, col in enumerate(row)])

class NaiveQueryResultWrapper(ExtQueryResultWrapper):
    def process_row(self, row):
        instance = self.model()
        for i, column, func in self.conv:
            setattr(instance, column, func(row[i]))
        instance.prepared()
        return instance

class DictQueryResultWrapper(ExtQueryResultWrapper):
    def process_row(self, row):
        res = {}
        for i, column, func in self.conv:
            res[column] = func(row[i])
        return res

class ModelQueryResultWrapper(QueryResultWrapper):
    def initialize(self, description):
        self.column_map, model_set = self.generate_column_map()
        self.join_list = self.generate_join_list(model_set)

    def generate_column_map(self):
        column_map = []
        models = set([self.model])
        for i, node in enumerate(self.column_meta):
            attr = conv = None
            if isinstance(node, Field):
                if isinstance(node, FieldProxy):
                    key = node._model_alias
                    constructor = node.model
                else:
                    key = constructor = node.model_class
                attr = node.name
                conv = node.python_value
            else:
                key = constructor = self.model
                if isinstance(node, Expression) and node._alias:
                    attr = node._alias
            column_map.append((key, constructor, attr, conv))
            models.add(key)

        return column_map, models

    def generate_join_list(self, models):
        join_list = []
        joins = self.join_meta
        stack = [self.model]
        while stack:
            current = stack.pop()
            if current not in joins:
                continue

            meta = current._meta
            for join in joins[current]:
                join_model = join.dest
                if join_model in models:
                    fk_field = meta.rel_for_model(join_model)
                    related_name = to_field = None
                    if fk_field is not None:
                        fk_name = fk_field.name
                        to_field = fk_field.to_field.name
                    else:
                        fk_field = meta.reverse_rel_for_model(join_model)
                        if fk_field is not None:
                            related_name = fk_field.related_name
                        if isinstance(join.on, Expression):
                            fk_name = join.on._alias or join.on.lhs.name
                        else:
                            # Patch the joined model using the name of the
                            # database table.
                            fk_name = join_model._meta.db_table

                    stack.append(join_model)
                    join_list.append((
                        current, fk_name, join_model, to_field, related_name))

        return join_list

    def process_row(self, row):
        collected = self.construct_instances(row)
        instances = self.follow_joins(collected)
        for i in instances:
            i.prepared()
        return instances[0]

    def construct_instances(self, row, keys=None):
        collected_models = {}
        for i, (key, constructor, attr, conv) in enumerate(self.column_map):
            if keys is not None and key not in keys:
                continue
            value = row[i]
            if key not in collected_models:
                collected_models[key] = constructor()
            instance = collected_models[key]
            if attr is None:
                attr = self.cursor.description[i][0]
            if conv is not None:
                value = conv(value)
            setattr(instance, attr, value)

        return collected_models

    def follow_joins(self, collected):
        prepared = [collected[self.model]]
        for (lhs, attr, rhs, to_field, related_name) in self.join_list:
            inst = collected[lhs]
            joined_inst = collected[rhs]

            # Can we populate a value on the joined instance using the current?
            if to_field is not None and attr in inst._data:
                if getattr(joined_inst, to_field) is None:
                    setattr(joined_inst, to_field, inst._data[attr])

            setattr(inst, attr, joined_inst)
            prepared.append(joined_inst)

        return prepared


class AggregateQueryResultWrapper(ModelQueryResultWrapper):
    def __init__(self, *args, **kwargs):
        self._row = []
        super(AggregateQueryResultWrapper, self).__init__(*args, **kwargs)

    def initialize(self, description):
        super(AggregateQueryResultWrapper, self).initialize(description)

        # Collect the set of all models queried.
        self.all_models = set()
        for key, _, _, _ in self.column_map:
            self.all_models.add(key)

        # Prepare data structure for analyzing unique rows.
        self.models_with_aggregate = set()
        self.back_references = {}
        for (src_model, _, dest_model, _, related_name) in self.join_list:
            if related_name:
                self.models_with_aggregate.add(src_model)
                self.back_references[dest_model] = (src_model, related_name)

        self.columns_to_compare = {}
        for idx, (_, model_class, col_name, _) in enumerate(self.column_map):
            if model_class in self.models_with_aggregate:
                self.columns_to_compare.setdefault(model_class, [])
                self.columns_to_compare[model_class].append((idx, col_name))

    def read_model_data(self, row):
        models = {}
        for model_class, column_data in self.columns_to_compare.items():
            models[model_class] = []
            for idx, col_name in column_data:
                models[model_class].append(row[idx])
        return models

    def iterate(self):
        if self._row:
            row = self._row.pop()
        else:
            row = self.cursor.fetchone()

        if not row:
            self._populated = True
            raise StopIteration
        elif not self._initialized:
            self.initialize(self.cursor.description)
            self._initialized = True

        identity_map = {}
        _constructed = self.construct_instances(row)
        primary_instance = _constructed[self.model]
        for model_class, instance in _constructed.items():
            identity_map[model_class] = {instance.get_id(): instance}

        model_data = self.read_model_data(row)
        while True:
            cur_row = self.cursor.fetchone()
            if cur_row is None:
                break

            duplicate_models = set()
            cur_row_data = self.read_model_data(cur_row)
            for model_class, data in cur_row_data.items():
                if model_data[model_class] == data:
                    duplicate_models.add(model_class)

            if not duplicate_models:
                self._row.append(cur_row)
                break

            different_models = self.all_models - duplicate_models

            new_instances = self.construct_instances(cur_row, different_models)
            for model_class, instance in new_instances.items():
                # Do not include any instances which are comprised solely of
                # NULL values.
                if [val for val in instance._data.values() if val is not None]:
                    identity_map[model_class][instance.get_id()] = instance

        stack = [self.model]
        while stack:
            current = stack.pop()
            if current not in self.join_meta:
                continue

            for join in self.join_meta[current]:
                foreign_key = current._meta.rel_for_model(join.dest)
                if foreign_key:
                    for pk, instance in identity_map[current].items():
                        joined_inst = identity_map[join.dest][
                            instance._data[foreign_key.name]]
                        setattr(instance, foreign_key.name, joined_inst)
                else:
                    backref = current._meta.reverse_rel_for_model(join.dest)
                    if not backref:
                        continue

                    attr_name = backref.related_name
                    for instance in identity_map[current].values():
                        setattr(instance, attr_name, [])

                    for pk, instance in identity_map[join.dest].items():
                        try:
                            joined_inst = identity_map[current][
                                instance._data[backref.name]]
                        except KeyError:
                            continue

                        getattr(joined_inst, attr_name).append(instance)

                stack.append(join.dest)

        return primary_instance


class Query(Node):
    """Base class representing a database query on one or more tables."""
    require_commit = True

    def __init__(self, model_class):
        super(Query, self).__init__()

        self.model_class = model_class
        self.database = model_class._meta.database

        self._dirty = True
        self._query_ctx = model_class
        self._joins = {self.model_class: []}  # Join graph as adjacency list.
        self._where = None

    def __repr__(self):
        sql, params = self.sql()
        return '%s %s %s' % (self.model_class, sql, params)

    def clone(self):
        query = type(self)(self.model_class)
        query.database = self.database
        return self._clone_attributes(query)

    def _clone_attributes(self, query):
        if self._where is not None:
            query._where = self._where.clone()
        query._joins = self._clone_joins()
        query._query_ctx = self._query_ctx
        return query

    def _clone_joins(self):
        return dict(
            (mc, list(j)) for mc, j in self._joins.items())

    def _add_query_clauses(self, initial, expressions, conjunction=None):
        reduced = reduce(operator.and_, expressions)
        if initial is None:
            return reduced
        conjunction = conjunction or operator.and_
        return conjunction(initial, reduced)

    @returns_clone
    def where(self, *expressions):
        self._where = self._add_query_clauses(self._where, expressions)

    @returns_clone
    def orwhere(self, *expressions):
        self._where = self._add_query_clauses(
            self._where, expressions, operator.or_)

    @returns_clone
    def join(self, model_class, join_type=None, on=None):
        if not self._query_ctx._meta.rel_exists(model_class) and on is None:
            raise ValueError('No foreign key between %s and %s' % (
                self._query_ctx, model_class))
        if on and isinstance(on, basestring):
            on = self._query_ctx._meta.fields[on]
        self._joins.setdefault(self._query_ctx, [])
        self._joins[self._query_ctx].append(Join(model_class, join_type, on))
        self._query_ctx = model_class

    @returns_clone
    def switch(self, model_class=None):
        """Change or reset the query context."""
        self._query_ctx = model_class or self.model_class

    def ensure_join(self, lm, rm, on=None):
        ctx = self._query_ctx
        for join in self._joins.get(lm, []):
            if join.dest == rm:
                return self
        return self.switch(lm).join(rm, on=on).switch(ctx)

    def convert_dict_to_node(self, qdict):
        accum = []
        joins = []
        relationship = (ForeignKeyField, ReverseRelationDescriptor)
        for key, value in sorted(qdict.items()):
            curr = self.model_class
            if '__' in key and key.rsplit('__', 1)[1] in DJANGO_MAP:
                key, op = key.rsplit('__', 1)
                op = DJANGO_MAP[op]
            else:
                op = OP_EQ
            for piece in key.split('__'):
                model_attr = getattr(curr, piece)
                if isinstance(model_attr, relationship):
                    curr = model_attr.rel_model
                    joins.append(model_attr)
            accum.append(Expression(model_attr, op, value))
        return accum, joins

    def filter(self, *args, **kwargs):
        # normalize args and kwargs into a new expression
        dq_node = Node()
        if args:
            dq_node &= reduce(operator.and_, [a.clone() for a in args])
        if kwargs:
            dq_node &= DQ(**kwargs)

        # dq_node should now be an Expression, lhs = Node(), rhs = ...
        q = deque([dq_node])
        dq_joins = set()
        while q:
            curr = q.popleft()
            if not isinstance(curr, Expression):
                continue
            for side, piece in (('lhs', curr.lhs), ('rhs', curr.rhs)):
                if isinstance(piece, DQ):
                    query, joins = self.convert_dict_to_node(piece.query)
                    dq_joins.update(joins)
                    expression = reduce(operator.and_, query)
                    # Apply values from the DQ object.
                    expression._negated = piece._negated
                    expression._alias = piece._alias
                    setattr(curr, side, expression)
                else:
                    q.append(piece)

        dq_node = dq_node.rhs

        query = self.clone()
        for field in dq_joins:
            if isinstance(field, ForeignKeyField):
                lm, rm = field.model_class, field.rel_model
                field_obj = field
            elif isinstance(field, ReverseRelationDescriptor):
                lm, rm = field.field.rel_model, field.rel_model
                field_obj = field.field
            query = query.ensure_join(lm, rm, field_obj)
        return query.where(dq_node)

    def compiler(self):
        return self.database.compiler()

    def sql(self):
        raise NotImplementedError

    def _execute(self):
        sql, params = self.sql()
        return self.database.execute_sql(sql, params, self.require_commit)

    def execute(self):
        raise NotImplementedError

    def scalar(self, as_tuple=False, convert=False):
        if convert:
            row = self.tuples().first()
        else:
            row = self._execute().fetchone()
        if row and not as_tuple:
            return row[0]
        else:
            return row

class RawQuery(Query):
    """
    Execute a SQL query, returning a standard iterable interface that returns
    model instances.
    """
    def __init__(self, model, query, *params):
        self._sql = query
        self._params = list(params)
        self._qr = None
        self._tuples = False
        self._dicts = False
        super(RawQuery, self).__init__(model)

    def clone(self):
        query = RawQuery(self.model_class, self._sql, *self._params)
        query._tuples = self._tuples
        query._dicts = self._dicts
        return query

    join = not_allowed('joining')
    where = not_allowed('where')
    switch = not_allowed('switch')

    @returns_clone
    def tuples(self, tuples=True):
        self._tuples = tuples

    @returns_clone
    def dicts(self, dicts=True):
        self._dicts = dicts

    def sql(self):
        return self._sql, self._params

    def execute(self):
        if self._qr is None:
            if self._tuples:
                ResultWrapper = TuplesQueryResultWrapper
            elif self._dicts:
                ResultWrapper = DictQueryResultWrapper
            else:
                ResultWrapper = NaiveQueryResultWrapper
            self._qr = ResultWrapper(self.model_class, self._execute(), None)
        return self._qr

    def __iter__(self):
        return iter(self.execute())

class SelectQuery(Query):
    _node_type = 'select_query'

    def __init__(self, model_class, *selection):
        super(SelectQuery, self).__init__(model_class)
        self.require_commit = self.database.commit_select
        self.__select(*selection)
        self._from = None
        self._group_by = None
        self._having = None
        self._order_by = None
        self._windows = None
        self._limit = None
        self._offset = None
        self._distinct = False
        self._for_update = (False, False)
        self._naive = False
        self._tuples = False
        self._dicts = False
        self._aggregate_rows = False
        self._alias = None
        self._qr = None

    def _clone_attributes(self, query):
        query = super(SelectQuery, self)._clone_attributes(query)
        query._explicit_selection = self._explicit_selection
        query._select = list(self._select)
        if self._from is not None:
            query._from = []
            for f in self._from:
                if isinstance(f, Node):
                    query._from.append(f.clone())
                else:
                    query._from.append(f)
        if self._group_by is not None:
            query._group_by = list(self._group_by)
        if self._having:
            query._having = self._having.clone()
        if self._order_by is not None:
            query._order_by = list(self._order_by)
        if self._windows is not None:
            query._windows = list(self._windows)
        query._limit = self._limit
        query._offset = self._offset
        query._distinct = self._distinct
        query._for_update = self._for_update
        query._naive = self._naive
        query._tuples = self._tuples
        query._dicts = self._dicts
        query._aggregate_rows = self._aggregate_rows
        query._alias = self._alias
        return query

    def _model_shorthand(self, args):
        accum = []
        for arg in args:
            if isinstance(arg, Node):
                accum.append(arg)
            elif isinstance(arg, Query):
                accum.append(arg)
            elif isinstance(arg, ModelAlias):
                accum.extend(arg.get_proxy_fields())
            elif isclass(arg) and issubclass(arg, Model):
                accum.extend(arg._meta.get_fields())
        return accum

    def compound_op(operator):
        def inner(self, other):
            supported_ops = self.model_class._meta.database.compound_operations
            if operator not in supported_ops:
                raise ValueError(
                    'Your database does not support %s' % operator)
            return CompoundSelect(self.model_class, self, operator, other)
        return inner
    __or__ = compound_op('UNION')
    __and__ = compound_op('INTERSECT')
    __sub__ = compound_op('EXCEPT')

    def __xor__(self, rhs):
        # Symmetric difference, should just be (self | rhs) - (self & rhs)...
        wrapped_rhs = self.model_class.select(SQL('*')).from_(
            EnclosedClause((self & rhs)).alias('_')).order_by()
        return (self | rhs) - wrapped_rhs

    def __select(self, *selection):
        self._explicit_selection = len(selection) > 0
        selection = selection or self.model_class._meta.get_fields()
        self._select = self._model_shorthand(selection)
    select = returns_clone(__select)

    @returns_clone
    def from_(self, *args):
        self._from = None
        if args:
            self._from = list(args)

    @returns_clone
    def group_by(self, *args):
        self._group_by = self._model_shorthand(args)

    @returns_clone
    def having(self, *expressions):
        self._having = self._add_query_clauses(self._having, expressions)

    @returns_clone
    def order_by(self, *args):
        self._order_by = list(args)

    @returns_clone
    def window(self, *windows):
        self._windows = list(windows)

    @returns_clone
    def limit(self, lim):
        self._limit = lim

    @returns_clone
    def offset(self, off):
        self._offset = off

    @returns_clone
    def paginate(self, page, paginate_by=20):
        if page > 0:
            page -= 1
        self._limit = paginate_by
        self._offset = page * paginate_by

    @returns_clone
    def distinct(self, is_distinct=True):
        self._distinct = is_distinct

    @returns_clone
    def for_update(self, for_update=True, nowait=False):
        self._for_update = (for_update, nowait)

    @returns_clone
    def naive(self, naive=True):
        self._naive = naive

    @returns_clone
    def tuples(self, tuples=True):
        self._tuples = tuples

    @returns_clone
    def dicts(self, dicts=True):
        self._dicts = dicts

    @returns_clone
    def aggregate_rows(self, aggregate_rows=True):
        self._aggregate_rows = aggregate_rows

    @returns_clone
    def alias(self, alias=None):
        self._alias = alias

    def annotate(self, rel_model, annotation=None):
        if annotation is None:
            annotation = fn.Count(rel_model._meta.primary_key).alias('count')
        query = self.clone()
        query = query.ensure_join(query._query_ctx, rel_model)
        if not query._group_by:
            query._group_by = [x.alias() for x in query._select]
        query._select = tuple(query._select) + (annotation,)
        return query

    def _aggregate(self, aggregation=None):
        if aggregation is None:
            aggregation = fn.Count(SQL('*'))
        query = self.order_by()
        query._select = [aggregation]
        return query

    def aggregate(self, aggregation=None, convert=True):
        return self._aggregate(aggregation).scalar(convert=convert)

    def count(self, clear_limit=False):
        if self._distinct or self._group_by or self._limit or self._offset:
            return self.wrapped_count(clear_limit=clear_limit)

        # defaults to a count() of the primary key
        return self.aggregate(convert=False) or 0

    def wrapped_count(self, clear_limit=False):
        clone = self.order_by()
        if clear_limit:
            clone._limit = clone._offset = None

        sql, params = clone.sql()
        wrapped = 'SELECT COUNT(1) FROM (%s) AS wrapped_select' % sql
        rq = self.model_class.raw(wrapped, *params)
        return rq.scalar() or 0

    def exists(self):
        clone = self.paginate(1, 1)
        clone._select = [SQL('1')]
        return bool(clone.scalar())

    def get(self):
        clone = self.paginate(1, 1)
        try:
            return clone.execute().next()
        except StopIteration:
            raise self.model_class.DoesNotExist(
                'Instance matching query does not exist:\nSQL: %s\nPARAMS: %s'
                % self.sql())

    def first(self):
        res = self.execute()
        res.fill_cache(1)
        try:
            return res._result_cache[0]
        except IndexError:
            pass

    def sql(self):
        return self.compiler().generate_select(self)

    def verify_naive(self):
        model_class = self.model_class
        for node in self._select:
            if isinstance(node, Field) and node.model_class != model_class:
                return False
        return True

    def get_query_meta(self):
        return (self._select, self._joins)

    def execute(self):
        if self._dirty or not self._qr:
            model_class = self.model_class
            query_meta = self.get_query_meta()
            if self._tuples:
                ResultWrapper = TuplesQueryResultWrapper
            elif self._dicts:
                ResultWrapper = DictQueryResultWrapper
            elif self._naive or not self._joins or self.verify_naive():
                ResultWrapper = NaiveQueryResultWrapper
            elif self._aggregate_rows:
                ResultWrapper = AggregateQueryResultWrapper
            else:
                ResultWrapper = ModelQueryResultWrapper
            self._qr = ResultWrapper(model_class, self._execute(), query_meta)
            self._dirty = False
            return self._qr
        else:
            return self._qr

    def __iter__(self):
        return iter(self.execute())

    def iterator(self):
        return iter(self.execute().iterator())

    def __getitem__(self, value):
        res = self.execute()
        if isinstance(value, slice):
            index = value.stop
        else:
            index = value
        if index is not None and index >= 0:
            index += 1
        res.fill_cache(index)
        return res._result_cache[value]

class CompoundSelect(SelectQuery):
    _node_type = 'compound_select_query'

    def __init__(self, model_class, lhs=None, operator=None, rhs=None):
        self.lhs = lhs
        self.operator = operator
        self.rhs = rhs
        super(CompoundSelect, self).__init__(model_class, [])

    def _clone_attributes(self, query):
        query = super(CompoundSelect, self)._clone_attributes(query)
        query.lhs = self.lhs
        query.operator = self.operator
        query.rhs = self.rhs
        return query

    def get_query_meta(self):
        return self.lhs.get_query_meta()


class UpdateQuery(Query):
    def __init__(self, model_class, update=None):
        self._update = update
        super(UpdateQuery, self).__init__(model_class)

    def _clone_attributes(self, query):
        query = super(UpdateQuery, self)._clone_attributes(query)
        query._update = dict(self._update)
        return query

    join = not_allowed('joining')

    def sql(self):
        return self.compiler().generate_update(self)

    def execute(self):
        return self.database.rows_affected(self._execute())

class InsertQuery(Query):
    def __init__(self, model_class, field_dict=None, rows=None,
                 fields=None, query=None):
        super(InsertQuery, self).__init__(model_class)

        self._upsert = False
        self._is_multi_row_insert = rows is not None or query is not None
        if rows is not None:
            self._rows = rows
        else:
            self._rows = [field_dict or {}]

        if query is None:
            self._defaults = self._get_default_values()
            self._valid_fields = (set(model_class._meta.fields.keys()) |
                                  set(model_class._meta.fields.values()))

        self._fields = fields
        self._query = query

    def _get_default_values(self):
        defaults = self.model_class._meta.get_default_dict()
        return dict(
            (self.model_class._meta.fields[f], v) for f, v in defaults.items())

    def _iter_rows(self):
        # Convert {'field_name': value} to {FieldName: value} / apply defaults.
        for row_dict in self._rows:
            field_row = {}
            field_row.update(self._defaults)
            for key in row_dict:
                if key not in self._valid_fields:
                    raise KeyError('"%s" is not a recognized field.' % key)
                elif key in self.model_class._meta.fields:
                    field = self.model_class._meta.fields[key]
                else:
                    field = key
                field_row[field] = row_dict[key]
            yield field_row

    def _clone_attributes(self, query):
        query = super(InsertQuery, self)._clone_attributes(query)
        query._rows = self._rows
        query._upsert = self._upsert
        query._is_multi_row_insert = self._is_multi_row_insert
        query._fields = self._fields
        query._query = self._query
        return query

    join = not_allowed('joining')
    where = not_allowed('where clause')

    @returns_clone
    def upsert(self, upsert=True):
        self._upsert = upsert

    def sql(self):
        return self.compiler().generate_insert(self)

    def execute(self):
        if self._is_multi_row_insert and self._query is None:
            if not self.database.insert_many:
                last_id = None
                for row in self._rows:
                    last_id = InsertQuery(self.model_class, row).execute()
                return last_id
        return self.database.last_insert_id(self._execute(), self.model_class)

class DeleteQuery(Query):
    join = not_allowed('joining')

    def sql(self):
        return self.compiler().generate_delete(self)

    def execute(self):
        return self.database.rows_affected(self._execute())


class PeeweeException(Exception): pass
class ImproperlyConfigured(PeeweeException): pass
class DatabaseError(PeeweeException): pass
class DataError(DatabaseError): pass
class IntegrityError(DatabaseError): pass
class InterfaceError(PeeweeException): pass
class InternalError(DatabaseError): pass
class NotSupportedError(DatabaseError): pass
class OperationalError(DatabaseError): pass
class ProgrammingError(DatabaseError): pass


class ExceptionWrapper(object):
    __slots__ = ['exceptions']

    def __init__(self, exceptions):
        self.exceptions = exceptions

    def __enter__(self): pass
    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            return
        if exc_type.__name__ in self.exceptions:
            new_type = self.exceptions[exc_type.__name__]
            reraise(new_type, new_type(*exc_value.args), traceback)


class Database(object):
    commit_select = False
    compiler_class = QueryCompiler
    compound_operations = ['UNION', 'INTERSECT', 'EXCEPT']
    distinct_on = False
    drop_cascade = True
    field_overrides = {}
    foreign_keys = True
    for_update = False
    for_update_nowait = False
    insert_many = True
    interpolation = '?'
    limit_max = None
    op_overrides = {}
    quote_char = '"'
    reserved_tables = []
    savepoints = True
    sequences = False
    subquery_delete_same_table = True
    window_functions = False

    exceptions = {
        'DatabaseError': DatabaseError,
        'DataError': DataError,
        'IntegrityError': IntegrityError,
        'InterfaceError': InterfaceError,
        'InternalError': InternalError,
        'NotSupportedError': NotSupportedError,
        'OperationalError': OperationalError,
        'ProgrammingError': ProgrammingError}

    def __init__(self, database, threadlocals=False, autocommit=True,
                 fields=None, ops=None, autorollback=False, **connect_kwargs):
        self.init(database, **connect_kwargs)

        if threadlocals:
            self.__local = threading.local()
        else:
            self.__local = type('DummyLocal', (object,), {})

        self._conn_lock = threading.Lock()
        self.autocommit = autocommit
        self.autorollback = autorollback

        self.field_overrides = merge_dict(self.field_overrides, fields or {})
        self.op_overrides = merge_dict(self.op_overrides, ops or {})

    def init(self, database, **connect_kwargs):
        self.deferred = database is None
        self.database = database
        self.connect_kwargs = connect_kwargs

    def exception_wrapper(self):
        return ExceptionWrapper(self.exceptions)

    def connect(self):
        with self._conn_lock:
            if self.deferred:
                raise Exception('Error, database not properly initialized '
                                'before opening connection')
            with self.exception_wrapper():
                self.__local.conn = self._connect(
                    self.database,
                    **self.connect_kwargs)
                self.__local.closed = False

    def close(self):
        with self._conn_lock:
            if self.deferred:
                raise Exception('Error, database not properly initialized '
                                'before closing connection')
            with self.exception_wrapper():
                self._close(self.__local.conn)
                self.__local.closed = True

    def get_conn(self):
        if not hasattr(self.__local, 'closed') or self.__local.closed:
            self.connect()
        return self.__local.conn

    def is_closed(self):
        return getattr(self.__local, 'closed', True)

    def get_cursor(self):
        return self.get_conn().cursor()

    def _close(self, conn):
        conn.close()

    def _connect(self, database, **kwargs):
        raise NotImplementedError

    @classmethod
    def register_fields(cls, fields):
        cls.field_overrides = merge_dict(cls.field_overrides, fields)

    @classmethod
    def register_ops(cls, ops):
        cls.op_overrides = merge_dict(cls.op_overrides, ops)

    def last_insert_id(self, cursor, model):
        if model._meta.auto_increment:
            return cursor.lastrowid

    def rows_affected(self, cursor):
        return cursor.rowcount

    def sql_error_handler(self, exception, sql, params, require_commit):
        return True

    def compiler(self):
        return self.compiler_class(
            self.quote_char, self.interpolation, self.field_overrides,
            self.op_overrides)

    def execute_sql(self, sql, params=None, require_commit=True):
        logger.debug((sql, params))
        with self.exception_wrapper():
            cursor = self.get_cursor()
            try:
                cursor.execute(sql, params or ())
            except Exception as exc:
                if self.get_autocommit() and self.autorollback:
                    self.rollback()
                if self.sql_error_handler(exc, sql, params, require_commit):
                    raise
            else:
                if require_commit and self.get_autocommit():
                    self.commit()
        return cursor

    def begin(self):
        pass

    def commit(self):
        self.get_conn().commit()

    def rollback(self):
        self.get_conn().rollback()

    def set_autocommit(self, autocommit):
        self.__local.autocommit = autocommit

    def get_autocommit(self):
        if not hasattr(self.__local, 'autocommit'):
            self.set_autocommit(self.autocommit)
        return self.__local.autocommit

    def push_transaction(self, transaction):
        if not hasattr(self.__local, 'transactions'):
            self.__local.transactions = []
        self.__local.transactions.append(transaction)

    def pop_transaction(self):
        self.__local.transactions.pop()

    def transaction_depth(self):
        return len(getattr(self.__local, 'transactions', ()))

    def transaction(self):
        return transaction(self)

    def commit_on_success(self, func):
        @wraps(func)
        def inner(*args, **kwargs):
            with self.transaction():
                return func(*args, **kwargs)
        return inner

    def savepoint(self, sid=None):
        if not self.savepoints:
            raise NotImplementedError
        return savepoint(self, sid)

    def get_tables(self):
        raise NotImplementedError

    def get_indexes_for_table(self, table):
        raise NotImplementedError

    def sequence_exists(self, seq):
        raise NotImplementedError

    def create_table(self, model_class, safe=False):
        qc = self.compiler()
        return self.execute_sql(*qc.create_table(model_class, safe))

    def create_tables(self, models, safe=False):
        create_model_tables(models, fail_silently=safe)

    def create_index(self, model_class, fields, unique=False):
        qc = self.compiler()
        if not isinstance(fields, (list, tuple)):
            raise ValueError('Fields passed to "create_index" must be a list '
                             'or tuple: "%s"' % fields)
        fobjs = [
            model_class._meta.fields[f] if isinstance(f, basestring) else f
            for f in fields]
        return self.execute_sql(*qc.create_index(model_class, fobjs, unique))

    def create_foreign_key(self, model_class, field, constraint=None):
        qc = self.compiler()
        return self.execute_sql(*qc.create_foreign_key(
            model_class, field, constraint))

    def create_sequence(self, seq):
        if self.sequences:
            qc = self.compiler()
            return self.execute_sql(*qc.create_sequence(seq))

    def drop_table(self, model_class, fail_silently=False, cascade=False):
        qc = self.compiler()
        return self.execute_sql(*qc.drop_table(
            model_class, fail_silently, cascade))

    def drop_tables(self, models, safe=False, cascade=False):
        drop_model_tables(models, fail_silently=safe, cascade=cascade)

    def drop_sequence(self, seq):
        if self.sequences:
            qc = self.compiler()
            return self.execute_sql(*qc.drop_sequence(seq))

    def extract_date(self, date_part, date_field):
        return fn.EXTRACT(Clause(date_part, R('FROM'), date_field))

    def truncate_date(self, date_part, date_field):
        return fn.DATE_TRUNC(SQL(date_part), date_field)

class SqliteDatabase(Database):
    drop_cascade = False
    foreign_keys = False
    insert_many = sqlite3 and sqlite3.sqlite_version_info >= (3, 7, 11, 0)
    limit_max = -1
    op_overrides = {
        OP_LIKE: 'GLOB',
        OP_ILIKE: 'LIKE',
    }

    def __init__(self, *args, **kwargs):
        self._journal_mode = kwargs.pop('journal_mode', None)
        super(SqliteDatabase, self).__init__(*args, **kwargs)

    def _connect(self, database, **kwargs):
        conn = sqlite3.connect(database, **kwargs)
        self._add_conn_hooks(conn)
        return conn

    def _add_conn_hooks(self, conn):
        conn.create_function('date_part', 2, _sqlite_date_part)
        conn.create_function('date_trunc', 2, _sqlite_date_trunc)
        conn.create_function('regexp', 2, _sqlite_regexp)
        if self._journal_mode:
            self.execute_sql('PRAGMA journal_mode=%s;' % self._journal_mode)

    def get_indexes_for_table(self, table):
        res = self.execute_sql('PRAGMA index_list(%s);' % self.quote(table))
        rows = sorted([(r[1], r[2] == 1) for r in res.fetchall()])
        return rows

    def get_tables(self):
        res = self.execute_sql('select name from sqlite_master where '
                               'type="table" order by name;')
        return [r[0] for r in res.fetchall()]

    def savepoint(self, sid=None):
        return savepoint_sqlite(self, sid)

    def extract_date(self, date_part, date_field):
        return fn.date_part(date_part, date_field)

    def truncate_date(self, date_part, date_field):
        return fn.strftime(SQLITE_DATE_TRUNC_MAPPING[date_part], date_field)

class PostgresqlDatabase(Database):
    commit_select = True
    distinct_on = True
    field_overrides = {
        'blob': 'BYTEA',
        'bool': 'BOOLEAN',
        'datetime': 'TIMESTAMP',
        'decimal': 'NUMERIC',
        'double': 'DOUBLE PRECISION',
        'primary_key': 'SERIAL',
        'uuid': 'UUID',
    }
    for_update = True
    for_update_nowait = True
    interpolation = '%s'
    op_overrides = {
        OP_REGEXP: '~',
    }
    reserved_tables = ['user']
    sequences = True
    window_functions = True

    register_unicode = True

    def _connect(self, database, **kwargs):
        if not psycopg2:
            raise ImproperlyConfigured('psycopg2 must be installed.')
        conn = psycopg2.connect(database=database, **kwargs)
        if self.register_unicode:
            pg_extensions.register_type(pg_extensions.UNICODE, conn)
            pg_extensions.register_type(pg_extensions.UNICODEARRAY, conn)
        return conn

    def last_insert_id(self, cursor, model):
        meta = model._meta
        schema = ''
        if meta.schema:
            schema = '%s.' % meta.schema

        if meta.primary_key.sequence:
            seq = meta.primary_key.sequence
        elif meta.auto_increment:
            seq = '%s_%s_seq' % (meta.db_table, meta.primary_key.db_column)
        else:
            seq = None

        if seq:
            cursor.execute("SELECT CURRVAL('%s\"%s\"')" % (schema, seq))
            result = cursor.fetchone()[0]
            if self.get_autocommit():
                self.commit()
            return result

    def get_indexes_for_table(self, table):
        res = self.execute_sql("""
            SELECT c2.relname, i.indisprimary, i.indisunique
            FROM
            pg_catalog.pg_class c,
            pg_catalog.pg_class c2,
            pg_catalog.pg_index i
            WHERE
            c.relname = %s AND c.oid = i.indrelid AND i.indexrelid = c2.oid
            ORDER BY i.indisprimary DESC, i.indisunique DESC, c2.relname""",
            (table,))
        return sorted([(r[0], r[1]) for r in res.fetchall()])

    def get_tables(self):
        res = self.execute_sql("""
            SELECT c.relname
            FROM pg_catalog.pg_class c
            LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
            WHERE c.relkind IN ('r', 'v', '')
                AND n.nspname NOT IN ('pg_catalog', 'pg_toast')
                AND pg_catalog.pg_table_is_visible(c.oid)
            ORDER BY c.relname""")
        return [row[0] for row in res.fetchall()]

    def sequence_exists(self, sequence):
        res = self.execute_sql("""
            SELECT COUNT(*)
            FROM pg_class, pg_namespace
            WHERE relkind='S'
                AND pg_class.relnamespace = pg_namespace.oid
                AND relname=%s""", (sequence,))
        return bool(res.fetchone()[0])

    def set_search_path(self, *search_path):
        path_params = ','.join(['%s'] * len(search_path))
        self.execute_sql('SET search_path TO %s' % path_params, search_path)

class MySQLDatabase(Database):
    commit_select = True
    compound_operations = ['UNION']
    drop_cascade = False
    field_overrides = {
        'bool': 'BOOL',
        'decimal': 'NUMERIC',
        'double': 'DOUBLE PRECISION',
        'float': 'FLOAT',
        'primary_key': 'INTEGER AUTO_INCREMENT',
        'text': 'LONGTEXT',
    }
    for_update = True
    for_update_nowait = False
    interpolation = '%s'
    limit_max = 2 ** 64 - 1  # MySQL quirk
    op_overrides = {
        OP_LIKE: 'LIKE BINARY',
        OP_ILIKE: 'LIKE',
        OP_XOR: 'XOR',
    }
    quote_char = '`'
    subquery_delete_same_table = False

    def _connect(self, database, **kwargs):
        if not mysql:
            raise ImproperlyConfigured('Either MySQLdb or PyMySQL must be installed.')
        conn_kwargs = {
            'charset': 'utf8',
            'use_unicode': True,
        }
        conn_kwargs.update(kwargs)
        if 'password' in conn_kwargs:
            conn_kwargs['passwd'] = conn_kwargs.pop('password')
        return mysql.connect(db=database, **conn_kwargs)

    def get_indexes_for_table(self, table):
        res = self.execute_sql('SHOW INDEXES IN `%s`;' % table)
        rows = sorted([(r[2], r[1] == 0) for r in res.fetchall()])
        return rows

    def get_tables(self):
        res = self.execute_sql('SHOW TABLES;')
        return [r[0] for r in res.fetchall()]

    def extract_date(self, date_part, date_field):
        return fn.EXTRACT(Clause(R(date_part), R('FROM'), date_field))

    def truncate_date(self, date_part, date_field):
        return fn.DATE_FORMAT(date_field, MYSQL_DATE_TRUNC_MAPPING[date_part])


class transaction(object):
    def __init__(self, db):
        self.db = db

    def _begin(self):
        self.db.begin()

    def commit(self):
        self.db.commit()

    def rollback(self):
        self.db.rollback()

    def __enter__(self):
        self._orig = self.db.get_autocommit()
        self.db.set_autocommit(False)
        if self.db.transaction_depth() == 0:
            self._begin()
        self.db.push_transaction(self)
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        try:
            if exc_type:
                self.rollback()
            elif self.db.transaction_depth() == 1:
                try:
                    self.commit()
                except:
                    self.rollback()
                    raise
        finally:
            self.db.set_autocommit(self._orig)
            self.db.pop_transaction()


class savepoint(object):
    def __init__(self, db, sid=None):
        self.db = db
        _compiler = db.compiler()
        self.sid = sid or 's' + uuid.uuid4().hex
        self.quoted_sid = _compiler.quote(self.sid)

    def _execute(self, query):
        self.db.execute_sql(query, require_commit=False)

    def commit(self):
        self._execute('RELEASE SAVEPOINT %s;' % self.quoted_sid)

    def rollback(self):
        self._execute('ROLLBACK TO SAVEPOINT %s;' % self.quoted_sid)

    def __enter__(self):
        self._orig_autocommit = self.db.get_autocommit()
        self.db.set_autocommit(False)
        self._execute('SAVEPOINT %s;' % self.quoted_sid)
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        try:
            if exc_type:
                self.rollback()
            else:
                try:
                    self.commit()
                except:
                    self.rollback()
                    raise
        finally:
            self.db.set_autocommit(self._orig_autocommit)

class savepoint_sqlite(savepoint):
    def __enter__(self):
        conn = self.db.get_conn()
        # For sqlite, the connection's isolation_level *must* be set to None.
        # The act of setting it, though, will break any existing savepoints,
        # so only write to it if necessary.
        if conn.isolation_level is not None:
            self._orig_isolation_level = conn.isolation_level
            conn.isolation_level = None
        else:
            self._orig_isolation_level = None
        return super(savepoint_sqlite, self).__enter__()

    def __exit__(self, exc_type, exc_val, exc_tb):
        try:
            return super(savepoint_sqlite, self).__exit__(
                exc_type, exc_val, exc_tb)
        finally:
            if self._orig_isolation_level is not None:
                self.db.get_conn().isolation_level = self._orig_isolation_level

class FieldProxy(Field):
    def __init__(self, alias, field_instance):
        self._model_alias = alias
        self.model = self._model_alias.model_class
        self.field_instance = field_instance

    def clone_base(self):
        return FieldProxy(self._model_alias, self.field_instance)

    def __getattr__(self, attr):
        if attr == 'model_class':
            return self._model_alias
        return getattr(self.field_instance, attr)

class ModelAlias(object):
    def __init__(self, model_class):
        self.__dict__['model_class'] = model_class

    def __getattr__(self, attr):
        model_attr = getattr(self.model_class, attr)
        if isinstance(model_attr, Field):
            return FieldProxy(self, model_attr)
        return model_attr

    def __setattr__(self, attr, value):
        raise AttributeError('Cannot set attributes on ModelAlias instances')

    def get_proxy_fields(self):
        return [
            FieldProxy(self, f) for f in self.model_class._meta.get_fields()]

    def select(self, *selection):
        query = SelectQuery(self, *selection)
        if self._meta.order_by:
            query = query.order_by(*self._meta.order_by)
        return query


class DoesNotExist(Exception): pass

if sqlite3:
    default_database = SqliteDatabase('peewee.db')
else:
    default_database = None

class ModelOptions(object):
    def __init__(self, cls, database=None, db_table=None, indexes=None,
                 order_by=None, primary_key=None, table_alias=None,
                 constraints=None, schema=None, **kwargs):
        self.model_class = cls
        self.name = cls.__name__.lower()
        self.fields = {}
        self.columns = {}
        self.defaults = {}

        self.database = database or default_database
        self.db_table = db_table
        self.indexes = list(indexes or [])
        self.order_by = order_by
        self.primary_key = primary_key
        self.table_alias = table_alias
        self.constraints = constraints
        self.schema = schema

        self.auto_increment = None
        self.rel = {}
        self.reverse_rel = {}

        for key, value in kwargs.items():
            setattr(self, key, value)
        self._additional_keys = set(kwargs.keys())

    def prepared(self):
        for field in self.fields.values():
            if field.default is not None:
                self.defaults[field] = field.default

        if self.order_by:
            norm_order_by = []
            for item in self.order_by:
                if isinstance(item, Field):
                    prefix = '-' if item._ordering == 'DESC' else ''
                    item = prefix + item.name
                field = self.fields[item.lstrip('-')]
                if item.startswith('-'):
                    norm_order_by.append(field.desc())
                else:
                    norm_order_by.append(field.asc())
            self.order_by = norm_order_by

    def get_default_dict(self):
        dd = {}
        for field, default in self.defaults.items():
            if callable(default):
                dd[field.name] = default()
            else:
                dd[field.name] = default
        return dd

    def get_sorted_fields(self):
        key = lambda i: i[1]._sort_key
        return sorted(self.fields.items(), key=key)

    def get_field_names(self):
        return [f[0] for f in self.get_sorted_fields()]

    def get_fields(self):
        return [f[1] for f in self.get_sorted_fields()]

    def get_field_index(self, field):
        for i, (field_name, field_obj) in enumerate(self.get_sorted_fields()):
            if field_name == field.name:
                return i
        return -1

    def rel_for_model(self, model, field_obj=None):
        for field in self.get_fields():
            if isinstance(field, ForeignKeyField) and field.rel_model == model:
                if field_obj is None or field_obj.name == field.name:
                    return field

    def reverse_rel_for_model(self, model):
        return model._meta.rel_for_model(self.model_class)

    def rel_exists(self, model):
        return self.rel_for_model(model) or self.reverse_rel_for_model(model)


class BaseModel(type):
    inheritable = set(['constraints', 'database', 'indexes', 'order_by',
                       'primary_key', 'schema'])

    def __new__(cls, name, bases, attrs):
        if not bases:
            return super(BaseModel, cls).__new__(cls, name, bases, attrs)

        meta_options = {}
        meta = attrs.pop('Meta', None)
        if meta:
            for k, v in meta.__dict__.items():
                if not k.startswith('_'):
                    meta_options[k] = v

        model_pk = getattr(meta, 'primary_key', None)
        parent_pk = None

        # inherit any field descriptors by deep copying the underlying field
        # into the attrs of the new model, additionally see if the bases define
        # inheritable model options and swipe them
        for b in bases:
            if not hasattr(b, '_meta'):
                continue

            base_meta = getattr(b, '_meta')
            if parent_pk is None:
                parent_pk = deepcopy(base_meta.primary_key)
            all_inheritable = cls.inheritable | base_meta._additional_keys
            for (k, v) in base_meta.__dict__.items():
                if k in all_inheritable and k not in meta_options:
                    meta_options[k] = v

            for (k, v) in b.__dict__.items():
                if k in attrs:
                    continue
                if isinstance(v, FieldDescriptor):
                    if not v.field.primary_key:
                        attrs[k] = deepcopy(v.field)

        # initialize the new class and set the magic attributes
        cls = super(BaseModel, cls).__new__(cls, name, bases, attrs)
        cls._meta = ModelOptions(cls, **meta_options)
        cls._data = None
        cls._meta.indexes = list(cls._meta.indexes)

        # replace fields with field descriptors, calling the add_to_class hook
        fields = []
        for name, attr in cls.__dict__.items():
            if isinstance(attr, Field):
                if attr.primary_key and model_pk:
                    raise ValueError('primary key is overdetermined.')
                elif attr.primary_key:
                    model_pk, pk_name = attr, name
                else:
                    fields.append((attr, name))

        if model_pk is None:
            if parent_pk:
                model_pk, pk_name = parent_pk, parent_pk.name
            else:
                model_pk, pk_name = PrimaryKeyField(primary_key=True), 'id'
        elif isinstance(model_pk, CompositeKey):
            pk_name = '_composite_key'

        model_pk.add_to_class(cls, pk_name)
        cls._meta.primary_key = model_pk
        cls._meta.auto_increment = (
            isinstance(model_pk, PrimaryKeyField) or bool(model_pk.sequence))

        for field, name in fields:
            field.add_to_class(cls, name)

        if not cls._meta.db_table:
            cls._meta.db_table = re.sub('[^\w]+', '_', cls.__name__.lower())

        # create a repr and error class before finalizing
        if hasattr(cls, '__unicode__'):
            setattr(cls, '__repr__', lambda self: '<%s: %r>' % (
                cls.__name__, self.__unicode__()))

        exc_name = '%sDoesNotExist' % cls.__name__
        exception_class = type(exc_name, (DoesNotExist,), {})
        cls.DoesNotExist = exception_class
        cls._meta.prepared()

        return cls

class Model(with_metaclass(BaseModel)):
    def __init__(self, *args, **kwargs):
        self._data = self._meta.get_default_dict()
        self._dirty = set()
        self._obj_cache = {} # cache of related objects

        for k, v in kwargs.items():
            setattr(self, k, v)

    @classmethod
    def alias(cls):
        return ModelAlias(cls)

    @classmethod
    def select(cls, *selection):
        query = SelectQuery(cls, *selection)
        if cls._meta.order_by:
            query = query.order_by(*cls._meta.order_by)
        return query

    @classmethod
    def update(cls, **update):
        fdict = dict((cls._meta.fields[f], v) for f, v in update.items())
        return UpdateQuery(cls, fdict)

    @classmethod
    def insert(cls, **insert):
        return InsertQuery(cls, insert)

    @classmethod
    def insert_many(cls, rows):
        return InsertQuery(cls, rows=rows)

    @classmethod
    def insert_from(cls, fields, query):
        return InsertQuery(cls, fields=fields, query=query)

    @classmethod
    def delete(cls):
        return DeleteQuery(cls)

    @classmethod
    def raw(cls, sql, *params):
        return RawQuery(cls, sql, *params)

    @classmethod
    def create(cls, **query):
        inst = cls(**query)
        inst.save(force_insert=True)
        inst.prepared()
        return inst

    @classmethod
    def get(cls, *query, **kwargs):
        sq = cls.select().naive()
        if query:
            sq = sq.where(*query)
        if kwargs:
            sq = sq.filter(**kwargs)
        return sq.get()

    @classmethod
    def get_or_create(cls, **kwargs):
        sq = cls.select().filter(**kwargs)
        try:
            return sq.get()
        except cls.DoesNotExist:
            return cls.create(**kwargs)

    @classmethod
    def filter(cls, *dq, **query):
        return cls.select().filter(*dq, **query)

    @classmethod
    def table_exists(cls):
        return cls._meta.db_table in cls._meta.database.get_tables()

    @classmethod
    def create_table(cls, fail_silently=False):
        if fail_silently and cls.table_exists():
            return

        db = cls._meta.database
        pk = cls._meta.primary_key
        if db.sequences and pk.sequence:
            if not db.sequence_exists(pk.sequence):
                db.create_sequence(pk.sequence)

        db.create_table(cls)
        cls._create_indexes()

    @classmethod
    def _fields_to_index(cls):
        fields = []
        for field in cls._meta.fields.values():
            if field.primary_key:
                continue
            requires_index = any((
                field.index,
                field.unique,
                isinstance(field, ForeignKeyField)))
            if requires_index:
                fields.append(field)
        return fields

    @classmethod
    def _create_indexes(cls):
        db = cls._meta.database
        for field in cls._fields_to_index():
            db.create_index(cls, [field], field.unique)

        if cls._meta.indexes:
            for fields, unique in cls._meta.indexes:
                db.create_index(cls, fields, unique)

    @classmethod
    def sqlall(cls):
        queries = []
        compiler = cls._meta.database.compiler()
        pk = cls._meta.primary_key
        if cls._meta.database.sequences and pk.sequence:
            queries.append(compiler.create_sequence(pk.sequence))
        queries.append(compiler.create_table(cls))
        for field in cls._fields_to_index():
            queries.append(compiler.create_index(cls, [field], field.unique))
        if cls._meta.indexes:
            for field_names, unique in cls._meta.indexes:
                fields = [cls._meta.fields[f] for f in field_names]
                queries.append(compiler.create_index(cls, fields, unique))
        return [sql for sql, _ in queries]

    @classmethod
    def drop_table(cls, fail_silently=False, cascade=False):
        cls._meta.database.drop_table(cls, fail_silently, cascade)

    @classmethod
    def _as_entity(cls):
        if cls._meta.schema:
            return Entity(cls._meta.schema, cls._meta.db_table)
        return Entity(cls._meta.db_table)

    def get_id(self):
        return getattr(self, self._meta.primary_key.name)

    def set_id(self, id):
        setattr(self, self._meta.primary_key.name, id)

    def pk_expr(self):
        return self._meta.primary_key == self.get_id()

    def prepared(self):
        pass

    def _prune_fields(self, field_dict, only):
        new_data = {}
        for field in only:
            if field.name in field_dict:
                new_data[field.name] = field_dict[field.name]
        return new_data

    def save(self, force_insert=False, only=None):
        field_dict = dict(self._data)
        pk_field = self._meta.primary_key
        if only:
            field_dict = self._prune_fields(field_dict, only)
        if self.get_id() is not None and not force_insert:
            if not isinstance(pk_field, CompositeKey):
                field_dict.pop(pk_field.name, None)
            else:
                field_dict = self._prune_fields(field_dict, self.dirty_fields)
            rows = self.update(**field_dict).where(self.pk_expr()).execute()
        else:
            pk = self.get_id()
            pk_from_cursor = self.insert(**field_dict).execute()
            if pk_from_cursor is not None:
                pk = pk_from_cursor
            self.set_id(pk)  # Do not overwrite current ID with a None value.
            rows = 1
        self._dirty.clear()
        return rows

    def is_dirty(self):
        return bool(self._dirty)

    @property
    def dirty_fields(self):
        return [f for f in self._meta.get_fields() if f.name in self._dirty]

    def dependencies(self, search_nullable=False):
        query = self.select().where(self.pk_expr())
        stack = [(type(self), query)]
        seen = set()

        while stack:
            klass, query = stack.pop()
            if klass in seen:
                continue
            seen.add(klass)
            for rel_name, fk in klass._meta.reverse_rel.items():
                rel_model = fk.model_class
                node = fk << query
                if not fk.null or search_nullable:
                    stack.append((rel_model, rel_model.select().where(node)))
                yield (node, fk)

    def delete_instance(self, recursive=False, delete_nullable=False):
        if recursive:
            dependencies = self.dependencies(delete_nullable)
            for query, fk in reversed(list(dependencies)):
                model = fk.model_class
                if fk.null and not delete_nullable:
                    model.update(**{fk.name: None}).where(query).execute()
                else:
                    model.delete().where(query).execute()
        return self.delete().where(self.pk_expr()).execute()

    def __eq__(self, other):
        return (
            other.__class__ == self.__class__ and
            self.get_id() is not None and
            other.get_id() == self.get_id())

    def __ne__(self, other):
        return not self == other


def prefetch_add_subquery(sq, subqueries):
    fixed_queries = [(sq, None)]
    for i, subquery in enumerate(subqueries):
        if not isinstance(subquery, Query) and issubclass(subquery, Model):
            subquery = subquery.select()
        subquery_model = subquery.model_class
        fkf = None
        for j in reversed(range(i + 1)):
            last_query = fixed_queries[j][0]
            fkf = subquery_model._meta.rel_for_model(last_query.model_class)
            if fkf:
                break
        if not fkf:
            raise AttributeError('Error: unable to find foreign key for '
                                 'query: %s' % subquery)
        inner_query = last_query.select(fkf.to_field)
        fixed_queries.append((subquery.where(fkf << inner_query), fkf))

    return fixed_queries

def prefetch(sq, *subqueries):
    if not subqueries:
        return sq
    fixed_queries = prefetch_add_subquery(sq, subqueries)

    deps = {}
    rel_map = {}
    for query, foreign_key_field in reversed(fixed_queries):
        query_model = query.model_class
        deps[query_model] = {}
        id_map = deps[query_model]
        has_relations = bool(rel_map.get(query_model))

        for result in query:
            if foreign_key_field:
                fk_val = result._data[foreign_key_field.name]
                id_map.setdefault(fk_val, [])
                id_map[fk_val].append(result)
            if has_relations:
                for rel_model, rel_fk in rel_map[query_model]:
                    rel_name = '%s_prefetch' % rel_fk.related_name
                    identifier = getattr(result, rel_fk.to_field.name)
                    rel_instances = deps[rel_model].get(identifier, [])
                    for inst in rel_instances:
                        setattr(inst, rel_fk.name, result)
                    setattr(result, rel_name, rel_instances)
        if foreign_key_field:
            rel_model = foreign_key_field.rel_model
            rel_map.setdefault(rel_model, [])
            rel_map[rel_model].append((query_model, foreign_key_field))

    return query

def create_model_tables(models, **create_table_kwargs):
    """Create tables for all given models (in the right order)."""
    for m in sort_models_topologically(models):
        m.create_table(**create_table_kwargs)

def drop_model_tables(models, **drop_table_kwargs):
    """Drop tables for all given models (in the right order)."""
    for m in reversed(sort_models_topologically(models)):
        m.drop_table(**drop_table_kwargs)

def sort_models_topologically(models):
    """Sort models topologically so that parents will precede children."""
    models = set(models)
    seen = set()
    ordering = []
    def dfs(model):
        if model in models and model not in seen:
            seen.add(model)
            for foreign_key in model._meta.reverse_rel.values():
                dfs(foreign_key.model_class)
            ordering.append(model)  # parent will follow descendants
    # order models by name and table initially to guarantee a total ordering
    names = lambda m: (m._meta.name, m._meta.db_table)
    for m in sorted(models, key=names, reverse=True):
        dfs(m)
    return list(reversed(ordering))  # want parents first in output ordering