def setter(obj, value):
    setattr(obj, proxy.value_attr, value)
return getter, setter


class C(Base):
    __tablename__ = 'c'
id = Column(Integer, primary_key=True)
id_b = Column(Integer, ForeignKey('b.id'))
b = relationship('B', backref='cs')
a = association_proxy('b', 'a', getset_factory=outer_join_accessor_factory)
Association_proxy с отношениями 0-n