SQLAlchemy - 具有额外列的自引用多对多关系

### 自引用多对多关系中的额外列实现

python
from sqlalchemy import create_engine, Column, Integer, String, ForeignKey, DateTime
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.ext.associationproxy import association_proxy
from datetime import datetime
Base = declarative_base()
class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    following = relationship(
        'UserRelationship',
        foreign_keys='UserRelationship.follower_id',
        backref='follower',
        cascade='all, delete-orphan'
    )
    followers = relationship(
        'UserRelationship',
        foreign_keys='UserRelationship.followed_id',
        backref='followed',
        cascade='all, delete-orphan'
    )
class UserRelationship(Base):
    __tablename__ = 'user_relationship'
    follower_id = Column(Integer, ForeignKey('users.id'), primary_key=True)
    followed_id = Column(Integer, ForeignKey('users.id'), primary_key=True)
    follow_date = Column(DateTime, default=datetime.utcnow)
    relationship_note = Column(String)

python
# 创建数据库引擎和表
engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)
# 创建会话
Session = sessionmaker(bind=engine)
session = Session()
# 创建用户
user1 = User(name='Alice')
user2 = User(name='Bob')
user3 = User(name='Charlie')
# 添加用户到数据库
session.add_all([user1, user2, user3])
session.commit()
# 创建用户关系并添加额外信息
user1.following.append(UserRelationship(followed=user2, relationship_note='Met at conference'))
user1.following.append(UserRelationship(followed=user3, relationship_note='Mutual interest in art'))
session.commit()