contains_eager 和joinedload 之间的SQLAlchemy 区别

Question

SQLAlchemycontains_eager和joinedload. 我阅读了关于 contains_eager的手册，以及关于 joinload 的手册。它们都可用于加载一对多相关行或多对一。

他们正在生成相同的 sql：

query = session.query(User).\
    outerjoin(adalias, User.addresses).\
    options(contains_eager(User.addresses, alias=adalias)).all()

...

SELECT
    users.user_id AS users_user_id,
    users.user_name AS users_user_name,
    adalias.address_id AS adalias_address_id,
    adalias.user_id AS adalias_user_id,
    adalias.email_address AS adalias_email_address,
    (...other columns...)
FROM users
LEFT OUTER JOIN email_addresses AS email_addresses_1
ON users.user_id = email_addresses_1.user_id


>>> jack = session.query(User).\
... options(joinedload(User.addresses)).\
... filter_by(name='jack').all()

SELECT
    addresses_1.id AS addresses_1_id,
    addresses_1.email_address AS addresses_1_email_address,
    addresses_1.user_id AS addresses_1_user_id,
    users.id AS users_id, users.name AS users_name,
    users.fullname AS users_fullname,
    users.password AS users_password
FROM users
LEFT OUTER JOIN addresses AS addresses_1
    ON users.id = addresses_1.user_id
WHERE users.name = ?
['jack']

有人可以展示更具体的代码示例吗？

Answer 1

不要忘记多对多和一对一。不同之处在于，contains_eager()您可以向 SQLA 指示一个或多个应用于填充关系的现有连接。通过这种方式，您还可以使用过滤后的子集进行填充。

joinedload() on the other hand goes to great lengths to make the joins required for populating entirely transparent and they should not affect the outcome of the original query, as is explained in The Zen of Joined Eager Loading. In other words you cannot use the joined relations for filtering etc.