如何使用flask sqlalchemy实现多对多关系？

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
db = SQLAlchemy(app)

# 创建中间表
association_table = db.Table('association',
    db.Column('user_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('role_id', db.Integer, db.ForeignKey('role.id'))
)

# 创建用户模型
class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    roles = db.relationship('Role', secondary=association_table, backref='users')

# 创建角色模型
class Role(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

# 创建数据库表
db.create_all()

# 添加多对多关系
user = User(name='John')
role = Role(name='Admin')
user.roles.append(role)

# 提交到数据库
db.session.add(user)
db.session.add(role)
db.session.commit()