python flask 多对多表查询功能

post_class=db.Table('post_class',
  db.Column('post_id',db.Integer(),db.ForeignKey('posts.id')),
  db.Column('classifa_id',db.Integer(),db.ForeignKey('fenlei.id')))
class Post(db.Model):#文章表
  __tablename__='posts'
  id=db.Column(db.Integer,primary_key=True,autoincrement=True)
  title=db.Column(db.String(255),unique=True)
  text=db.Column(db.Text())
  publish_date=db.Column(db.DateTime,default=datetime.datetime.now())
  user_id=db.Column(db.Integer,db.ForeignKey('users.id'))
  is_recomment=db.Column(db.Boolean,default=False)
  comments = db.relationship(
    'Comment',
    backref='posts',
    lazy='dynamic')
  tag = db.relationship(
    'Tag',
    secondary=posts_tags,
    backref=db.backref('posts', lazy='dynamic')
  )
  classname=db.relationship('Classifa',
    secondary=post_class,
    backref=db.backref('posts'))
  def __repr__(self):
    return "<Model Post `{}`>".format(self.title)
class Classifa(db.Model):#分类
  __tablename__='fenlei'
  id=db.Column(db.Integer(),primary_key=True)
  name=db.Column(db.String(64))
  def __repr__(self):
    return self.name

data=Classifa.query.filter_by(name='数据库').first()
 data_post=data.posts