FLASK在删除对象的父级后无法删除该对象

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 Parent(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    children = db.relationship('Child', cascade='all, delete', backref='parent')

class Child(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'))

@app.route('/delete_parent/<int:parent_id>', methods=['DELETE'])
def delete_parent(parent_id):
    parent = Parent.query.get(parent_id)
    if parent:
        # 删除与父级对象相关联的子级对象
        Child.query.filter_by(parent_id=parent_id).delete()
        # 删除父级对象
        db.session.delete(parent)
        db.session.commit()
        return '删除成功'
    else:
        return '父级对象不存在'

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