如何在SQLAlchemy ORM上实现自引用多对多关系,并引用相同的属性？

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

Base = declarative_base()

friends_association_table = Table('friends_association', Base.metadata,
    Column('user_id', Integer, ForeignKey('users.id')),
    Column('friend_id', Integer, ForeignKey('users.id'))
)

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    friends = relationship('User', secondary=friends_association_table,
                           primaryjoin=id==friends_association_table.c.user_id,
                           secondaryjoin=id==friends_association_table.c.friend_id,
                           backref='friends')

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine = create_engine('数据库连接字符串')
Session = sessionmaker(bind=engine)
session = Session()

Base.metadata.create_all(engine)

# 创建用户
user1 = User(name='User 1')
user2 = User(name='User 2')
user3 = User(name='User 3')

# 建立关系
user1.friends.append(user2)
user1.friends.append(user3)
user2.friends.append(user3)

# 提交到数据库
session.add_all([user1, user2, user3])
session.commit()

# 查询用户的朋友列表
user = session.query(User).filter_by(name='User 1').first()
friends = user.friends

for friend in friends:
    print(friend.name)