lazy 决定了 SQLAlchemy 什么时候从数据库中加载数据:，有如下四个值:(其实还有个noload不常用) select: (which is the default) means that SQLAlchemy will load the data as necessary in one go using a standard select statement. joined: tells SQLAlchemy to load the relationship in the same query as the parent using a JOIN statement. subquery: works like ‘joined’ but instead SQLAlchemy will use a subquery. dynamic : is special and useful if you have many items. Instead of loading the items SQLAlchemy will return another query object which you can further refine before loading the items. This is usually what you want if you expect more than a handful of items for this relationship
from sqlalchemy import Column,Integer,Text,DECIMAL,String,ForeignKeyfrom sqlalchemy import create_enginefrom sqlalchemy.ext.declarative import declarative_basefrom sqlalchemy.orm import sessionmakerfrom sqlalchemy.orm import relationshipfrom sqlalchemy import DateTime
# 1.创建连接connect_msg = 'mysql+pymysql://root:2008.Cn123@192.168.0.101/test'engine=create_engine(connect_msg)
# 2.创建SessionSession = sessionmaker(bind=engine)# 实例化 Sessionsession = Session()
# 3.创建基类Base = declarative_base()
# 4.创建默认的一对多关系表, 默认的加载方式是 lazy='select'class User1(Base): __tablename__ = 'test_user1'
id = Column(Integer, nullable=False, primary_key=True) name = Column(String(45))
articles = relationship('Article1', back_populates='user')
class Article1(Base): __tablename__ = 'test_article1'
id = Column(Integer, nullable=False, primary_key=True) name = Column(String(45)) create_time = Column(DateTime) u_id = Column(Integer, ForeignKey('test_user1.id'))
user = relationship('User1', back_populates='articles')
# 5.创建懒加载 lazy='dynamic' 的一对多关系.class User2(Base): __tablename__ = 'test_user2'
id = Column(Integer, nullable=False, primary_key=True) name = Column(String(45))
articles = relationship('Article2', back_populates='user', lazy='dynamic')
class Article2(Base): __tablename__ = 'test_article2'
id = Column(Integer, nullable=False, primary_key=True) name = Column(String(45)) create_time = Column(DateTime) u_id = Column(Integer, ForeignKey('test_user2.id'))
user = relationship('User2', back_populates='articles')
# 6.创建表Base.metadata.drop_all(engine)Base.metadata.create_all(engine)
# 7.添加数据u1 = User1(id=1, name='Jack')
for i in range(1,100): a1 = Article1(id=i,name='xxxx',create_time='201{}-1{}-2{} 11:2{}:1{}'.format(i%2,i%2,i%2,i%2, i%2), u_id=1) u1.articles.append(a1) session.add(a1)
u2= User2(id=1, name='Jack')
for i in range(1,100): a1 = Article2(id=i,name='xxxx',create_time='201{}-1{}-2{} 11:2{}:1{}'.format(i%2,i%2,i%2,i%2,i%2), u_id=1) u2.articles.append(a1) session.add(a1)
session.commit()
首先,使用默认创建的一对多关系,查看Model 上的关系.
# 拿到 父表 映射 类 中的 第一条实例,也就是 第一个作者q1 = session.query(User1).first()
# 通过关系的映射 relationship 来查看子表中第一个作者写的所有文章q1.articles
"""SELECT test_article1.id AS test_article1_id, test_article1.name AS test_article1_name, test_article1.create_time AS test_article1_create_time, test_article1.u_id AS test_article1_u_id FROM test_article1 WHERE 1 = test_article1.u_id"""
然后对照mysql 的查询日志,查看它查找的SQL 语句
$ tail -10 /mysql_data/localhost_log...SELECT test_article1.id AS test_article1_id, test_article1.name AS test_article1_name, test_article1.create_time AS test_article1_create_time, test_article1.u_id AS test_article1_u_id FROM test_article1 WHERE 1 = test_article1.u_id
>>> from sqlalchemy.orm.dynamic import AppenderQuery>>> help(AppenderQuery)
Help on class AppenderQuery in module sqlalchemy.orm.dynamic:class AppenderQuery(AppenderMixin, sqlalchemy.orm.query.Query)
# 查询一个实例q3 = test2.filter(Article2.create_time=='2011-11-21 11:21:11').filter(Article2.id==99).all()print(type(q3))print(q3)
"""SELECT test_article2.id AS test_article2_id, test_article2.name AS test_article2_name, test_article2.create_time AS test_article2_create_time, test_article2.u_id AS test_article2_u_id FROM test_article2 WHERE 1 = test_article2.u_id AND test_article2.create_time = '2011-11-21 11:21:11' AND test_article2.id = 99"""
然后对照mysql 的查询日志,查看它查找的SQL 语句
$ tail -10 /mysql_data/localhost_log...SELECT test_article2.id AS test_article2_id, test_article2.name AS test_article2_name, test_article2.create_time AS test_article2_create_time, test_article2.u_id AS test_article2_u_id FROM test_article2 WHERE 1 = test_article2.u_id AND test_article2.create_time = '2011-11-21 11:21:11' AND test_article2.id = 99
SELECT anon_1.test_user3_id AS anon_1_test_user3_id, anon_1.test_user3_name AS anon_1_test_user3_name, test_article3_1.id AS test_article3_1_id, test_article3_1.name AS test_article3_1_name, test_article3_1.create_time AS test_article3_1_create_time, test_article3_1.u_id AS test_article3_1_u_idFROM (SELECT test_user3.id AS test_user3_id, test_user3.name AS test_user3_nameFROM test_user3 LIMIT 1) AS anon_1 LEFT OUTER JOIN test_article3 AS test_article3_1 ON anon_1.test_user3_id = test_article3_1.u_id
