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backref和back_populates在表示两个表之间的关系时，很有用。

查看backref的文档，

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class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(Integer, primary_key=True)
    name = db.Column(String(10))
    addresses = relationship("Address", backref="user")

    def __repr__(self):
        return u'<user id={0}, name={1}>'.format(self.id, self.name).encode('utf-8')


class Address(db.Model):
    __tablename__ = 'addresses'
    id = db.Column(Integer, primary_key=True)
    email = db.Column(String(64))
    user_id = db.Column(Integer, ForeignKey('users.id'))

    def __repr__(self):
        return '<address id={0}, email={1} user_id={2}>'.format(self.id, self.email, self.user_id)

之后在命令行中，可以得到如下结果

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>>> u = User(name='Bob')
>>> db.session.add(u)
>>> db.session.commit()
>>> u.addresses
[]
>>> a = Address(email='bob@163.com', user_id=u.id)
>>> db.session.add(a)
>>> db.session.commit()
>>> u.addresses
[<address id=2, email=bob@163.com user_id=2>]
>>> a.user
<user id=2, name=Bob>

即通过u.addresses可以访问到用户的addresses, 而a.user可以访问到用户。注意到u.addresses返回的是列表，而a.user返回的是单个元素，即User与Address是一对多的关系。

也可以使用back_populates实现相同的功能，

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class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(Integer, primary_key=True)
    name = db.Column(String(10))
    addresses = relationship("Address", back_populates = "user")

    def __repr__(self):
        return u'<user id={0}, name={1}>'.format(self.id, self.name).encode('utf-8')


class Address(db.Model):
    __tablename__ = 'addresses'
    id = db.Column(Integer, primary_key=True)
    email = db.Column(String(64))
    user_id = db.Column(Integer, ForeignKey('users.id'))
    user = relationship("User", back_populates="addresses")

    def __repr__(self):
        return '<address id={0}, email={1} user_id={2}>'.format(self.id, self.email, self.user_id)

在交互环境中，得到如下结果

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>>> u = User(name='Clack')
>>> db.session.add(u)
>>> db.session.commit()
>>> u.addresses
[]
>>> a = Address(emial='Clack@163.com', user_id=u.id)
>>> a = Address(email='Clack@163.com', user_id=u.id)
>>> db.session.add(a)
>>> db.session.commit()
>>> u.addresses
[<address id=3, email=Clack@163.com user_id=3>]
>>> a.user
<user id=3, name=Clack>

从文档中得知，back_populates是用来取代backref的，虽然backref也是一直支持使用。倾向于使用back_populates, 因为它比backref更直接明了。

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