使用Flask-SQLAlchemy时多对多遇到问题，请帮忙看看

Question

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.name

class 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