aggregation聚集.md

Xiang Wang @ 2018-11-27 14:22:07

aggregation

• models

  from django.db import models
  
  class Author(models.Model):  # 作者
      name = models.CharField(max_length=100)
      age = models.IntegerField()
  
  class Publisher(models.Model):  # 出版商
      name = models.CharField(max_length=300)
  
  class Book(models.Model):  # 几个作者一起出一本书, 在一个出版商卖
      name = models.CharField(max_length=300)
      pages = models.IntegerField()
      price = models.DecimalField(max_digits=10, decimal_places=2)
      rating = models.FloatField()
      authors = models.ManyToManyField(Author)
      publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)
      pubdate = models.DateField()
  
  class Store(models.Model):  # 一个卖各种书的书店
      name = models.CharField(max_length=300)
      books = models.ManyToManyField(Book)
  

Cheat Sheet

```
from django.db.models import Avg
Book.objects.all().aggregate(Avg('price'))
above_5 = Count('book', filter=Q(book__rating__gt=5))
below_5 = Count('book', filter=Q(book__rating__lte=5))
pubs = Publisher.objects.annotate(below_5=below_5).annotate(above_5=above_5)
pubs[0].above_5
```

Generating aggregates over a QuerySet

Generating aggregates for each item in a QuerySet

• Combining multiple aggregations

Joins and aggregates

Aggregations and other QuerySet clauses