Flask 自关联表的错误：InvalidRequestError: Multiple classes found for path "Follow" in t

[Yambo1992]

#自引用多对多关系表
#这是models.py 中的Follow类
class Follow(db.Model):
    __tablename__ = 'followss'

    follower_id = db.Column(db.Integer, db.ForeignKey('users.id'),
                            primary_key=True)
    followed_id = db.Column(db.Integer, db.ForeignKey('users.id'),
                            primary_key=True)
    timestamp = db.Column(db.DateTime, default=datetime.datetime.utcnow)

#这是用户User类，筛选了主要可能有错的地方
class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True, index=True)
    password_hash = db.Column(db.String(128))
    confirmed = db.Column(db.Boolean, default=False)
    name = db.Column(db.String(64))

    followed = db.relationship('Follow',
                               foreign_keys=[Follow.follower_id],
                               backref=db.backref('follower', lazy='joined'),
                               lazy='dynamic', cascade='all, delete-orphan'）

    followers = db.relationship('Follow',
                                foreign_keys=[Follow.followed_id],
                                backref=db.backref('followed', lazy='joined'),
                                lazy='dynamic',
                                cascade='all, delete-orphan')

    def __init__(self, **kwargs):
        super(User, self).__init__(**kwargs)
       
    def follow(self, user):
        if not self.is_following(user):
            f = Follow(follower=self, followed=user)
            db.session.add(f)

    def unfollow(self, user):
        f = self.followed.filter_by(followed_id=user.id).first()
        if f:
            db.session.delete(f)

    def is_following(self, user):
        return self.followed.filter_by(
            followed_id=user.id).first() is not None

    def is_followed_by(self, user):
        return self.followers.filter_by(
            follower_id=user.id).first() is not None

#报错如下：
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/flask_sqlalchemy/__init__.py", line 512, in __get__
    mapper = orm.class_mapper(type)
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/base.py", line 421, in class_mapper
    mapper = _inspect_mapped_class(class_, configure=configure)
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/base.py", line 400, in _inspect_mapped_class
    mapper._configure_all()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1272, in _configure_all
    configure_mappers()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 3029, in configure_mappers
    mapper._post_configure_properties()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1828, in _post_configure_properties
    prop.init()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/interfaces.py", line 184, in init
    self.do_init()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1655, in do_init
    self._process_dependent_arguments()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1712, in _process_dependent_arguments
    self.target = self.mapper.mapped_table
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 767, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1628, in mapper
    argument = self.argument()
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 281, in __call__
    x = eval(self.arg, globals(), self._dict)
  File "<string>", line 1, in <module>
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/util/_collections.py", line 729, in __missing__
    self[key] = val = self.creator(key)
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 262, in _access_cls
    return _determine_container(key, cls._decl_class_registry[key])
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 246, in _determine_container
    value = value.attempt_get([], key)
  File "/home/yambo1992/projects/blog_1/flaskypr/env0/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 91, in attempt_get
    (".".join(path + [key]))
InvalidRequestError: Multiple classes found for path "Follow" in the registry of this declarative base.
 Please use a fully module-qualified path.