在添加数据SQL之前检查数据是否在数据库中

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

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(String, unique=True)

engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

def add_user(username):
    # 检查用户是否存在
    existing_user = session.query(User).filter_by(username=username).first()
    if existing_user:
        print(f"用户 {username} 已存在。")
        return False
    
    # 添加新用户
    new_user = User(username=username)
    session.add(new_user)
    session.commit()
    print(f"用户 {username} 添加成功。")
    return True

# 使用示例
add_user("john_doe")