SQLAlchemy使用笔记－－SQLAlchemy ORM（三）

参考：
http://docs.sqlalchemy.org/en/rel_1_0/orm/tutorial.html#eager-loading

预先加载

前面的代码，因为是lazy load，当我们调用User.addresses 时，sqlalchemy才会发出sql语句去取addresses，
比如：

query = session.query(User).all()
for user in query:
    print(user.addresses)

如果有10个用户，for循环10次，就会发10个取user中address信息的sql请求，这样很没有效率。
可以使用预先加载，在一个sql请求中吧User.addresses 都取出来。

subquery 加载

看例子：

users = session.query(User).options(subqueryload(User.addresses)).all()

发出的sql

SELECT users.id AS users_id, users.name AS users_name, users.fullname AS users_fullname, users.password AS users_password
FROM users;

SELECT addresses.id AS addresses_id, addresses.email_address AS addresses_email_address, addresses.user_id AS addresses_user_id, anon_1.users_id AS anon_1_users_id
FROM (SELECT users.id AS users_id
FROM users) AS anon_1 JOIN addresses ON anon_1.users_id = addresses.user_id ORDER BY anon_1.users_id, addresses.id;

当取出users时，会发出2个sql，第一个sql是取出所有user的基本信息，第二个sql时取出所有user的address信息。

joinedload 加载

joinedload 加载会使用 LEFT OUTER JOIN 来加载信息

users = session.query(User).options(joinedload(User.addresses)).all()

SELECT users.id AS users_id, users.name AS users_name, users.fullname AS users_fullname, users.password AS users_password, 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
FROM users LEFT OUTER JOIN addresses AS addresses_1 ON users.id = addr;

明确的使用join

当已经明确大join了User.addresses 后，可以使用contains_eager来加载address的信息。

users = session.query(User).join(User.addresses).options(contains_eager(User.addresses)).all()