创建一个包含4个类的flask模型，其中包含一个1:M连接和一个涉及User类的M:M连接

pip install flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = '数据库连接地址'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    classes = db.relationship('Class1', backref='user', lazy=True)

class Class1(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    class2s = db.relationship('Class2', backref='class1', lazy=True)

class Class2(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    class1_id = db.Column(db.Integer, db.ForeignKey('class1.id'), nullable=False)
    class3s = db.relationship('Class3', backref='class2', lazy=True)

class Class3(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    class2_id = db.Column(db.Integer, db.ForeignKey('class2.id'), nullable=False)

if __name__ == '__main__':
    db.create_all()
    app.run()

python app.py