Question

使用SQLAlchemy，我无法确定如何映射以下关系：用户可以跟随＆＃39;由其他用户，从而成为一个领导者＆＃39;与＆＃39;粉丝＆＃39;。

class User( Base ):
    __tablename__ = 'users'
    id = Column( Integer, primary_key = True )
    followers = relationship( 'User', primaryjoin = 'and_( User.id == UserFollower.leader_id )', cascade = 'all' )
    leaders = relationship( 'User', primaryjoin = 'and_( User.id == UserFollower.follower_id )', cascade = 'all' )

class UserFollower( Base ):
    __tablename__ = 'users_followers'
    leader_id = Column( Integer, ForeignKey( 'users.id' ), primary_key = True )
    follower_id = Column( Integer, ForeignKey( 'users.id' ), primary_key = True )
    follower = relationship( 'User', uselist = False, foreign_keys = [ follower_id ] )
    leader = relationship( 'User', uselist = False, foreign_keys = [ leader_id ] )

从文档中，我认为这样的声明会有

some_user.followers

执行＆＃39;用户＆＃39;和＆＃39; users_followers＆＃39;表格＆＃39; users.id = users_followers.leader_id＆＃39;其中＆＃39; user.id = some_user.id＆＃39;。

然而，这会产生错误（类似于How to specify table relationships in SQLAlchemy with multi-level/multiple joins?）。虽然建议的解决方案允许我解决这个问题，但它阻止了使用级联＆＃39; SQLAlchemy关系的属性，这样当删除用户时，相应的UserFollower对象也会被删除。

总结：如何将关系连接到具有多个外键的表引用相同的键并仍然保留级联（删除）？

Answer 1

如果您阅读SQLAlchemy文档的Self-Referential Many-to-Many Relationship部分，您将意识到您拥有经典的多对多关系。如果您根据文档定义此关系，级联将正常工作：

users_followers = Table(
    'users_followers', Base.metadata,
    Column('leader_id', ForeignKey('users.id'), primary_key=True),
    Column('follower_id', ForeignKey('users.id'), primary_key=True),
)


class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    followers = relationship(
        'User',
        secondary=users_followers,
        primaryjoin=id == users_followers.c.leader_id,
        secondaryjoin=id == users_followers.c.follower_id,
        backref="leaders",
        cascade='all',
    )

SQLAlchemy关系加入多个外键

1 个答案: