====== 第三十四章：ORM与SQLAlchemy ======

===== 本章目标 =====

完成本章学习后，你将能够：
  * 理解ORM概念
  * 使用SQLAlchemy定义模型
  * 执行CRUD操作
  * 处理关系映射

===== SQLAlchemy基础 =====

<code python>
from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

# 定义模型
class User(Base):
    __tablename__ = 'users'
    
    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    email = Column(String(100))
    
    def __repr__(self):
        return f"<User(name='{self.name}', email='{self.email}')>"

# 创建引擎和会话
engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
</code>

===== CRUD操作 =====

<code python>
# 创建
new_user = User(name='Alice', email='alice@example.com')
session.add(new_user)
session.commit()

# 查询
user = session.query(User).filter_by(name='Alice').first()
users = session.query(User).filter(User.age > 18).all()

# 更新
user.email = 'new@example.com'
session.commit()

# 删除
session.delete(user)
session.commit()
</code>

===== 关系映射 =====

<code python>
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship

class Post(Base):
    __tablename__ = 'posts'
    
    id = Column(Integer, primary_key=True)
    title = Column(String(100))
    user_id = Column(Integer, ForeignKey('users.id'))
    user = relationship("User", back_populates="posts")

User.posts = relationship("Post", back_populates="user")
</code>

===== 本章练习 =====

1. 定义多表关系模型
2. 实现复杂查询
3. 使用Alembic迁移数据库

下一章：[[python_course:chapter35|第三十五章：Flask Web开发]]