SQLAlchemy：尝试保存非唯一值后重新保存模型的唯一字段

使用SQLAlchemy处理模型中唯一字段的重复保存问题

python
from sqlalchemy import create_engine, Column, String, Integer, event
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
Base = declarative_base()
class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(String, unique=True)
# 创建数据库引擎和表
engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
# 插入初始数据
session = Session(engine)
user1 = User(username='john_doe')
session.add(user1)
session.commit()
# 定义事件监听器
@event.listens_for(User, 'before_insert')
@event.listens_for(User, 'before_update')
def check_unique(mapper, connection, target):
    existing_user = session.query(User).filter_by(username=target.username).first()
    if existing_user and existing_user.id != target.id:
        # 存在重复的用户名，执行自定义操作
        print(f"Username '{target.username}' already exists!")
# 尝试保存具有相同用户名的用户
user2 = User(username='john_doe')
session.add(user2)
session.commit()
session.close()