如果在User中使用relationship定义addresses属性的话，

addresses = relationship('Address')
则我们可以直接在User对象中通过addresses属性获得指定用户的所有地址

2 例子

>>> from sqlalchemy import Column, Integer, String

>>> class User(Base):

...     __tablename__ = 'users'

...

...     id = Column(Integer, primary_key=True)

...     name = Column(String)

...     fullname = Column(String)

...     password = Column(String)

...

...     def __repr__(self):

...        return "<User(name='%s', fullname='%s', password='%s')>" % ( self.name, self.fullname, self.password)

>>> from sqlalchemy import ForeignKey

>>> from sqlalchemy.orm import relationship, backref

>>> class Address(Base):

...     __tablename__ = 'addresses'

...     id = Column(Integer, primary_key=True)

...     email_address = Column(String, nullable=False)

...     user_id = Column(Integer, ForeignKey('users.id'))

...

...     user = relationship("User", backref=backref('addresses', order_by=id))

...

...     def __repr__(self):

...         return "<Address(email_address='%s')>" % self.email_address

ForeignKey表示，Addresses.user_id列的值应该等于users.id列中的值，即，users的主键
relationship(), 它告诉 ORM ,Address类本身应该使用属性Address.user链接到User类
relationship()的参数中有一个称为backref()的relationship()的子函数，反向提供详细的信息, 即在users中添加User对应的Address对象的集合，保存在User.addresses中
两个互补关系, Address.user和User.addresses被称为一个双向关系，并且这是SQLAlchemy ORM的一个关键特性