raise error when improper use (#47)

Changes:
- set the engine to None after disconnect. This prevents an incorrect
reuse.
- raise RuntimeError when db is not connected but connections are
  created.

databasez/core/database.py

@@ -449,6 +449,8 @@ async def drop_all(self, meta: MetaData, **kwargs: typing.Any) -> None:
 
     @multiloop_protector(False)
     def connection(self) -> Connection:
+        if not self.is_connected:
+            raise RuntimeError("Database is not connected")
         if self.force_rollback:
             return typing.cast(Connection, self._global_connection)
 

databasez/sqlalchemy.py

@@ -295,5 +295,6 @@ async def connect(self, database_url: DatabaseURL, **options: typing.Any) -> Non
 
     async def disconnect(self) -> None:
         engine = self.engine
+        self.engine = None
         assert engine is not None, "database is not initialized"
         await engine.dispose()