Python Flask-SQLAlchemy报sqlalchemy.exc.NoForeignKeysError是什么，,谢谢大家。代码如下，总是

Python Flask-SQLAlchemy报sqlalchemy.exc.NoForeignKeysError是什么，,谢谢大家。代码如下，总是

谢谢大家。
代码如下，总是会提示这个错误：

sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship Problem.tags - there are no foreign keys linking these tables via secondary table 'problem_tags'.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify 'primaryjoin' and 'secondaryjoin' expressions.

tags_table = db.Table('problem_tags',    db.Column('problem_id', db.Integer, db.ForeignKey('problem.id')),    db.Column('tag_id', db.Integer, db.ForeignKey('problemtag.id'))    )class ProblemTag(db.Model):    id=db.Column(db.Integer, primary_key=True)    name=db.Column(db.String(80))    problems = db.relationship('Problem',secondary=tags_table,                           backref=db.backref('tags', lazy='dynamic'))    def __init__(self,name):        self.name=name    def __repr__(self):        return "<Tag %r>" % self.nameclass Problem(db.Model):    id=db.Column(db.Integer, primary_key=True)    title=db.Column(db.String(80))    tags = db.relationship('ProblemTag',secondary=tags_table,                           backref=db.backref('problems', lazy='dynamic'))    def __init__(self,title,tags):        self.title=title        self.tags=tags    def __repr__(self):        return "<Problem %r>" % self.title

tags_table = db.Table('problem_tags',                      db.Column('problem_id', db.Integer, db.ForeignKey('problem.id')),                      db.Column('tag_id', db.Integer, db.ForeignKey('problemtag.id')))class ProblemTag(db.Model):    __tablename__ = 'problemtag'    id = db.Column(db.Integer, primary_key=True)    name = db.Column(db.String(80))    problems = db.relationship('Problem', secondary=tags_table,                               backref=db.backref('tags', lazy='dynamic'))    def __init__(self, name):        self.name = name    def __repr__(self):        return "<Tag %r>" % self.nameclass Problem(db.Model):    __tablename__ = 'problem'    id = db.Column(db.Integer, primary_key=True)    title = db.Column(db.String(80))    def __init__(self, title, tags):        self.title = title        self.tags = tags    def __repr__(self):        return "<Problem %r>" % self.title

编橙之家文章，