SQLAlchemy：joinedload 和 subqueryload 行为之间的区别
SQLAlchemy: Difference between joinedload and subqueryload behavior
我发现 SQLAlchemy 的 joinedload, and subqueryload, specifically with how they handle with_polymorphic 之间有一个惊人的区别。使用 subqueryload 时，我无法在 subqueryload 中包含的关系的子 class 上急切地加载关系，而 joinedload 似乎可以很好地处理这个问题。
例如，假设我有一个 User class，其中有许多 Shape 个对象。一个 Rectangle 是一个形状，并说它有一个 Widget 与之相关。在查询 User class 时，我希望能够构建我的查询选项，以便急切地获取用户的所有矩形小部件，并且我想将形状作为子查询进行查询（而不是联合加载）以减少返回结果集中的重复数据量。我可以通过加入加载 shapes 关系来让它工作，但我不能让它作为子查询工作。有什么想法吗？
示例，在代码中：
import logging
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import (
backref,
joinedload,
relationship,
scoped_session,
sessionmaker,
subqueryload,
with_polymorphic)
from sqlalchemy import (
create_engine,
Column,
Integer,
ForeignKey,
MetaData,
String)
logging.basicConfig()
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
Base = declarative_base()
class User(Base):
__tablename__ = "users"
id = Column(Integer, primary_key=True)
class Shape(Base):
__tablename__ = "shapes"
__mapper_args__ = {"polymorphic_on": "type"}
id = Column(Integer, primary_key=True)
type = Column(String, nullable=False)
owner_id = Column(Integer, ForeignKey("users.id"))
owner = relationship("User", backref=backref("shapes"), cascade="all")
class Rectangle(Shape):
__tablename__ = "shapes_rectangles"
__mapper_args__ = {"polymorphic_identity": "RECT"}
id = Column(Integer, ForeignKey("shapes.id"), primary_key=True)
length = Column(Integer)
width = Column(Integer)
widget_id = Column(Integer, ForeignKey("widgets.id"))
widget = relationship("Widget", cascade="all")
class Widget(Base):
__tablename__ = "widgets"
id = Column(Integer, primary_key=True)
engine = create_engine("sqlite://", echo=True)
metadata = MetaData()
with engine.begin() as connection:
metadata = Base.metadata
metadata.create_all(connection)
session = scoped_session(sessionmaker(bind=engine))
user = User()
widget = Widget()
rectangle = Rectangle(owner=user, widget=widget, length=10, width=5)
session.add_all([user, widget, rectangle])
session.commit()
shapes_load = subqueryload(
User.shapes.of_type(
with_polymorphic(Shape, "*", aliased=True)))
session.remove()
query = session.query(User) .options(shapes_load.joinedload(Rectangle.widget))
logger.info("##########subqueryload begin")
user = query.one()
logger.info("##########accessing widget")
user.shapes[0].widget
logger.info("##########subqueryload end")
shapes_load = joinedload(
User.shapes.of_type(
with_polymorphic(Shape, "*", aliased=True)))
session.remove()
query = session.query(User).options(shapes_load.joinedload(Rectangle.widget))
logger.info("##########joinedload begin")
user = query.one()
logger.info("##########accessing widget")
user.shapes[0].widget
logger.info("##########joinedload end")
相关输出：
INFO:__main__:##########subqueryload begin
2017-12-19 23:03:36,826 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
2017-12-19 23:03:36,827 INFO sqlalchemy.engine.base.Engine SELECT users.id AS users_id
FROM users
INFO:sqlalchemy.engine.base.Engine:SELECT users.id AS users_id
FROM users
2017-12-19 23:03:36,827 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
2017-12-19 23:03:36,829 INFO sqlalchemy.engine.base.Engine SELECT anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon
_1_shapes_owner_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length, anon_1.shapes_rectangles_wid
th AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id, anon_2.users_id AS anon_2_users_id
FROM (SELECT users.id AS users_id
FROM users) AS anon_2 JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON anon_2.users_id = anon_1.shapes_owner_id ORDER BY anon_2.users_id
INFO:sqlalchemy.engine.base.Engine:SELECT anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon_1_shapes_owner_id, anon
_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length, anon_1.shapes_rectangles_width AS anon_1_shapes_rect
angles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id, anon_2.users_id AS anon_2_users_id
FROM (SELECT users.id AS users_id
FROM users) AS anon_2 JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON anon_2.users_id = anon_1.shapes_owner_id ORDER BY anon_2.users_id
2017-12-19 23:03:36,829 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
INFO:__main__:##########accessing widget
2017-12-19 23:03:36,831 INFO sqlalchemy.engine.base.Engine SELECT widgets.id AS widgets_id
FROM widgets
WHERE widgets.id = ?
INFO:sqlalchemy.engine.base.Engine:SELECT widgets.id AS widgets_id
FROM widgets
WHERE widgets.id = ?
2017-12-19 23:03:36,831 INFO sqlalchemy.engine.base.Engine (1,)
INFO:sqlalchemy.engine.base.Engine:(1,)
INFO:__main__:##########subqueryload end
2017-12-19 23:03:36,832 INFO sqlalchemy.engine.base.Engine ROLLBACK
INFO:sqlalchemy.engine.base.Engine:ROLLBACK
INFO:__main__:##########joinedload begin
2017-12-19 23:03:36,837 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
2017-12-19 23:03:36,838 INFO sqlalchemy.engine.base.Engine SELECT users.id AS users_id, anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.s
hapes_owner_id AS anon_1_shapes_owner_id, widgets_1.id AS widgets_1_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_sh
apes_rectangles_length, anon_1.shapes_rectangles_width AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id
FROM users LEFT OUTER JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON users.id = anon_1.shapes_owner_id LEFT OUTER JOIN widgets AS widgets_1 ON widget
s_1.id = anon_1.shapes_rectangles_widget_id
INFO:sqlalchemy.engine.base.Engine:SELECT users.id AS users_id, anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon_1
_shapes_owner_id, widgets_1.id AS widgets_1_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length,
anon_1.shapes_rectangles_width AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id
FROM users LEFT OUTER JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON users.id = anon_1.shapes_owner_id LEFT OUTER JOIN widgets AS widgets_1 ON widget
s_1.id = anon_1.shapes_rectangles_widget_id
2017-12-19 23:03:36,839 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
INFO:__main__:##########accessing widget
INFO:__main__:##########joinedload end
我在使用 with_polymorphic 时发现了一个错误。此补丁 [1] 修复了 subqueryload 查询，因此它在访问 user.shapes[0].widget 时 不会 发出查询（这是预期的行为）。
在 SQLAlchemy 1.3 版中，原始测试用例失败并出现如下异常：
ArgumentError: Attribute "Rectangle.widget" does not link from element "with_polymorphic(Shape, [])".
[1]
@@ -72,12 +72,13 @@ rectangle = Rectangle(owner=user, widget=widget, length=10, width=5)
session.add_all([user, widget, rectangle])
session.commit()
-shapes_load = subqueryload(
- User.shapes.of_type(
- with_polymorphic(Shape, "*", aliased=True)))
+polymorphic_shape = with_polymorphic(Shape, "*", aliased=True)
+shapes_load = subqueryload(User.shapes.of_type(polymorphic_shape))
session.remove()
-query = session.query(User) .options(shapes_load.joinedload(Rectangle.widget))
+query = session.query(User).options(
+ shapes_load.joinedload(polymorphic_shape.Rectangle.widget)
+)
logger.info("##########subqueryload begin")
user = query.one()
logger.info("##########accessing widget")
@@ -85,11 +86,13 @@ user.shapes[0].widget
logger.info("##########subqueryload end")
shapes_load = joinedload(
- User.shapes.of_type(
- with_polymorphic(Shape, "*", aliased=True)))
+ User.shapes.of_type(with_polymorphic(Shape, "*", aliased=True))
+)
session.remove()
-query = session.query(User).options(shapes_load.joinedload(Rectangle.widget))
+query = session.query(User).options(
+ shapes_load.joinedload(polymorphic_shape.Rectangle.widget)
+)
logger.info("##########joinedload begin")
user = query.one()
logger.info("##########accessing widget")
我发现 SQLAlchemy 的 joinedload, and subqueryload, specifically with how they handle with_polymorphic 之间有一个惊人的区别。使用 subqueryload 时，我无法在 subqueryload 中包含的关系的子 class 上急切地加载关系，而 joinedload 似乎可以很好地处理这个问题。
例如，假设我有一个 User class，其中有许多 Shape 个对象。一个 Rectangle 是一个形状，并说它有一个 Widget 与之相关。在查询 User class 时，我希望能够构建我的查询选项，以便急切地获取用户的所有矩形小部件，并且我想将形状作为子查询进行查询（而不是联合加载）以减少返回结果集中的重复数据量。我可以通过加入加载 shapes 关系来让它工作，但我不能让它作为子查询工作。有什么想法吗？
示例，在代码中：
import logging
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import (
backref,
joinedload,
relationship,
scoped_session,
sessionmaker,
subqueryload,
with_polymorphic)
from sqlalchemy import (
create_engine,
Column,
Integer,
ForeignKey,
MetaData,
String)
logging.basicConfig()
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
Base = declarative_base()
class User(Base):
__tablename__ = "users"
id = Column(Integer, primary_key=True)
class Shape(Base):
__tablename__ = "shapes"
__mapper_args__ = {"polymorphic_on": "type"}
id = Column(Integer, primary_key=True)
type = Column(String, nullable=False)
owner_id = Column(Integer, ForeignKey("users.id"))
owner = relationship("User", backref=backref("shapes"), cascade="all")
class Rectangle(Shape):
__tablename__ = "shapes_rectangles"
__mapper_args__ = {"polymorphic_identity": "RECT"}
id = Column(Integer, ForeignKey("shapes.id"), primary_key=True)
length = Column(Integer)
width = Column(Integer)
widget_id = Column(Integer, ForeignKey("widgets.id"))
widget = relationship("Widget", cascade="all")
class Widget(Base):
__tablename__ = "widgets"
id = Column(Integer, primary_key=True)
engine = create_engine("sqlite://", echo=True)
metadata = MetaData()
with engine.begin() as connection:
metadata = Base.metadata
metadata.create_all(connection)
session = scoped_session(sessionmaker(bind=engine))
user = User()
widget = Widget()
rectangle = Rectangle(owner=user, widget=widget, length=10, width=5)
session.add_all([user, widget, rectangle])
session.commit()
shapes_load = subqueryload(
User.shapes.of_type(
with_polymorphic(Shape, "*", aliased=True)))
session.remove()
query = session.query(User) .options(shapes_load.joinedload(Rectangle.widget))
logger.info("##########subqueryload begin")
user = query.one()
logger.info("##########accessing widget")
user.shapes[0].widget
logger.info("##########subqueryload end")
shapes_load = joinedload(
User.shapes.of_type(
with_polymorphic(Shape, "*", aliased=True)))
session.remove()
query = session.query(User).options(shapes_load.joinedload(Rectangle.widget))
logger.info("##########joinedload begin")
user = query.one()
logger.info("##########accessing widget")
user.shapes[0].widget
logger.info("##########joinedload end")
相关输出：
INFO:__main__:##########subqueryload begin
2017-12-19 23:03:36,826 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
2017-12-19 23:03:36,827 INFO sqlalchemy.engine.base.Engine SELECT users.id AS users_id
FROM users
INFO:sqlalchemy.engine.base.Engine:SELECT users.id AS users_id
FROM users
2017-12-19 23:03:36,827 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
2017-12-19 23:03:36,829 INFO sqlalchemy.engine.base.Engine SELECT anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon
_1_shapes_owner_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length, anon_1.shapes_rectangles_wid
th AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id, anon_2.users_id AS anon_2_users_id
FROM (SELECT users.id AS users_id
FROM users) AS anon_2 JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON anon_2.users_id = anon_1.shapes_owner_id ORDER BY anon_2.users_id
INFO:sqlalchemy.engine.base.Engine:SELECT anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon_1_shapes_owner_id, anon
_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length, anon_1.shapes_rectangles_width AS anon_1_shapes_rect
angles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id, anon_2.users_id AS anon_2_users_id
FROM (SELECT users.id AS users_id
FROM users) AS anon_2 JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON anon_2.users_id = anon_1.shapes_owner_id ORDER BY anon_2.users_id
2017-12-19 23:03:36,829 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
INFO:__main__:##########accessing widget
2017-12-19 23:03:36,831 INFO sqlalchemy.engine.base.Engine SELECT widgets.id AS widgets_id
FROM widgets
WHERE widgets.id = ?
INFO:sqlalchemy.engine.base.Engine:SELECT widgets.id AS widgets_id
FROM widgets
WHERE widgets.id = ?
2017-12-19 23:03:36,831 INFO sqlalchemy.engine.base.Engine (1,)
INFO:sqlalchemy.engine.base.Engine:(1,)
INFO:__main__:##########subqueryload end
2017-12-19 23:03:36,832 INFO sqlalchemy.engine.base.Engine ROLLBACK
INFO:sqlalchemy.engine.base.Engine:ROLLBACK
INFO:__main__:##########joinedload begin
2017-12-19 23:03:36,837 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
2017-12-19 23:03:36,838 INFO sqlalchemy.engine.base.Engine SELECT users.id AS users_id, anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.s
hapes_owner_id AS anon_1_shapes_owner_id, widgets_1.id AS widgets_1_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_sh
apes_rectangles_length, anon_1.shapes_rectangles_width AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id
FROM users LEFT OUTER JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON users.id = anon_1.shapes_owner_id LEFT OUTER JOIN widgets AS widgets_1 ON widget
s_1.id = anon_1.shapes_rectangles_widget_id
INFO:sqlalchemy.engine.base.Engine:SELECT users.id AS users_id, anon_1.shapes_id AS anon_1_shapes_id, anon_1.shapes_type AS anon_1_shapes_type, anon_1.shapes_owner_id AS anon_1
_shapes_owner_id, widgets_1.id AS widgets_1_id, anon_1.shapes_rectangles_id AS anon_1_shapes_rectangles_id, anon_1.shapes_rectangles_length AS anon_1_shapes_rectangles_length,
anon_1.shapes_rectangles_width AS anon_1_shapes_rectangles_width, anon_1.shapes_rectangles_widget_id AS anon_1_shapes_rectangles_widget_id
FROM users LEFT OUTER JOIN (SELECT shapes.id AS shapes_id, shapes.type AS shapes_type, shapes.owner_id AS shapes_owner_id, shapes_rectangles.id AS shapes_rectangles_id, shapes_
rectangles.length AS shapes_rectangles_length, shapes_rectangles.width AS shapes_rectangles_width, shapes_rectangles.widget_id AS shapes_rectangles_widget_id
FROM shapes LEFT OUTER JOIN shapes_rectangles ON shapes.id = shapes_rectangles.id) AS anon_1 ON users.id = anon_1.shapes_owner_id LEFT OUTER JOIN widgets AS widgets_1 ON widget
s_1.id = anon_1.shapes_rectangles_widget_id
2017-12-19 23:03:36,839 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
INFO:__main__:##########accessing widget
INFO:__main__:##########joinedload end
我在使用 with_polymorphic 时发现了一个错误。此补丁 [1] 修复了 subqueryload 查询，因此它在访问 user.shapes[0].widget 时 不会 发出查询（这是预期的行为）。
在 SQLAlchemy 1.3 版中，原始测试用例失败并出现如下异常：
ArgumentError: Attribute "Rectangle.widget" does not link from element "with_polymorphic(Shape, [])".
[1]
@@ -72,12 +72,13 @@ rectangle = Rectangle(owner=user, widget=widget, length=10, width=5)
session.add_all([user, widget, rectangle])
session.commit()
-shapes_load = subqueryload(
- User.shapes.of_type(
- with_polymorphic(Shape, "*", aliased=True)))
+polymorphic_shape = with_polymorphic(Shape, "*", aliased=True)
+shapes_load = subqueryload(User.shapes.of_type(polymorphic_shape))
session.remove()
-query = session.query(User) .options(shapes_load.joinedload(Rectangle.widget))
+query = session.query(User).options(
+ shapes_load.joinedload(polymorphic_shape.Rectangle.widget)
+)
logger.info("##########subqueryload begin")
user = query.one()
logger.info("##########accessing widget")
@@ -85,11 +86,13 @@ user.shapes[0].widget
logger.info("##########subqueryload end")
shapes_load = joinedload(
- User.shapes.of_type(
- with_polymorphic(Shape, "*", aliased=True)))
+ User.shapes.of_type(with_polymorphic(Shape, "*", aliased=True))
+)
session.remove()
-query = session.query(User).options(shapes_load.joinedload(Rectangle.widget))
+query = session.query(User).options(
+ shapes_load.joinedload(polymorphic_shape.Rectangle.widget)
+)
logger.info("##########joinedload begin")
user = query.one()
logger.info("##########accessing widget")