sqlalchemy:多重关系(通过关联对象实现多对多)

Question

我正在尝试这样做:

class Foo(Base):
    id = Column(Integer, primary_key=True)

class Bar(Foo):
    id = Column(Integer, primary_key=True)

class FooBarAssociation(Base):
    foo_id = Column(Integer, ForeignKey('foo_table.id'))
    bar_id = Column(Integer, ForeignKey('bar_table.id'))

    foo = relationship(Foo, backref=...)
    bar = relationship(Bar, backref=...)

...但是我得到这样的错误:

Could not determine join condition between parent/child tables on relationship FooBarAssociation.foo.  Specify a 'primaryjoin' expression.  If this is a many-to-many relationship, 'secondaryjoin' is needed as well.

我试过在关系声明中指定 foreign_keys 和 primary_join-s，但都是徒劳。帮助？从 Foo 继承 Bar 是在搞乱我吗？

谢谢!

Answer 1

以下应该有效(正是错误告诉的内容:缺少 primaryjoin):

class FooBarAssociation(Base):
    foo_id = Column(Integer, ForeignKey('foo_table.id'), primary_key = True, )
    bar_id = Column(Integer, ForeignKey('bar_table.id'), ForeignKey('foo_table.id'), primary_key = True, )

    foo = relationship(Foo, backref="bars", primaryjoin=(foo_id==Foo.id))
    bar = relationship(Bar, backref="foos", primaryjoin=(bar_id==Bar.id))

如您所见，bar_id 列上有两个外键。由于继承，这可能是必需的，或者您可能会删除一个。但是，如果您不存储除多对多关系之外的任何其他信息，那么您可能会考虑 Association Proxy相反。