如何从数据库中删除SQLAlchemy多对多孤儿？

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    groups = relationship("Group", secondary="user_group", back_populates="users")

class Group(Base):
    __tablename__ = 'groups'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    users = relationship("User", secondary="user_group", back_populates="groups")

class UserGroup(Base):
    __tablename__ = 'user_group'
    user_id = Column(Integer, ForeignKey('users.id'), primary_key=True)
    group_id = Column(Integer, ForeignKey('groups.id'), primary_key=True)

# 创建数据库连接
engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

# 删除孤儿记录的函数
def delete_orphans():
    # 查询所有在user_group表中但不在任何users或groups表中的记录
    orphans = session.query(UserGroup).filter(
        ~UserGroup.user_id.in_(session.query(User.id)),
        ~UserGroup.group_id.in_(session.query(Group.id))
    ).all()
    
    # 删除这些孤儿记录
    for orphan in orphans:
        session.delete(orphan)
    
    session.commit()

# 调用函数删除孤儿记录
delete_orphans()