python sql炼金术多种关系

我想在这里建立这样的关系:

所以我的模型是这样定义的:

role_user = db.Table('role_user',
        db.Column('user_id', db.Integer, db.ForeignKey('user.id')),
        db.Column('role_id', db.Integer, db.ForeignKey('role.id')))

permission_role = db.Table('permission_role',
        db.Column('permission_id', db.Integer, db.ForeignKey('permission.id')),
        db.Column('role_id', db.Integer, db.ForeignKey('role.id')))


class role(db.Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(255))
    display_name = db.Column(db.String(255))
    description = db.Column(db.String(255))



class permission(db.Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(255))
    display_name = db.Column(db.String(255))
    description = db.Column(db.String(255))
    costam2 = db.relationship('role', secondary=permission_role, backref=db.backref('permissions', lazy='dynamic'))


class user(db.Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(255))
    email = db.Column(db.String(255))
    password = db.Column(db.String(255))
    costam1 = db.relationship('role', secondary=role_user, backref=db.backref('users', lazy='dynamic'))

我正在将记录添加到数据库中,如下所示:

admin = permission(name='admin', display_name='Admin', description='Admin')
operator = permission(name='operator', display_name='Operator', description='Operator')
ordinary_user = permission(name='ordinary_user', display_name='Ordinary user', description='Ordinary user')
db.session.add_all([admin, operator, ordinary_user])

user1 = user(name='user1', email='user1@user.pl', password='user1pass')
user2 = user(name='user2', email='user2@user.pl', password='user2pass')
user3 = user(name='user3', email='user3@user.pl', password='user3pass')
user4 = user(name='user4', email='user4@user.pl', password='user4pass')
db.session.add_all([user1, user2, user3, user4])

role1 = role(name='role1', display_name='role1', description='role1')
role2 = role(name='role2', display_name='role2', description='role2')
role3 = role(name='role3', display_name='role3', description='role3')
role4 = role(name='role4', display_name='role4', description='role4')
db.session.add_all([role1, role2, role3, role4])

到目前为止一切都很好。当我试图把它们联系起来时,问题就出来了,例如:

user1.permissions.append(admin)

结果是: AttributeError: 'user' object has no attribute 'permissions'

我在定义这些模型时是否犯了什么错误?可能是backref错了?