如何在Flask Sqlalchemy中更新外键值？

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'your_database_uri'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    role_id = db.Column(db.Integer, db.ForeignKey('role.id'))

class Role(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

# 查询旧角色和新角色的对象
old_role = Role.query.filter_by(name='旧角色').first()
new_role = Role.query.filter_by(name='新角色').first()

# 查询要更新的用户对象
user = User.query.filter_by(name='要更新的用户名').first()

# 更新外键值
user.role_id = new_role.id

# 提交更改到数据库
db.session.commit()