194 lines
6.4 KiB
Python
194 lines
6.4 KiB
Python
"""第4-3课示例：使用SQLAlchemy 2.x完成ORM增删改查和事务回滚。"""
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from decimal import Decimal
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from pathlib import Path
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import tomllib
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from sqlalchemy import Numeric, String, URL, create_engine, delete, select
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from sqlalchemy.exc import SQLAlchemyError
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from sqlalchemy.orm import DeclarativeBase, Mapped, Session, mapped_column, sessionmaker
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CONFIG_PATH = Path(__file__).with_name("config.toml")
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class Base(DeclarativeBase):
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"""保存本课所有ORM模型共享的映射元数据。"""
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class Book(Base):
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"""把Python图书对象映射到course_orm_book表。"""
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__tablename__ = "course_orm_book"
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id: Mapped[int] = mapped_column(primary_key=True)
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isbn: Mapped[str] = mapped_column(String(30), unique=True, nullable=False)
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title: Mapped[str] = mapped_column(String(100), nullable=False)
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author: Mapped[str] = mapped_column(String(50), nullable=False)
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price: Mapped[Decimal] = mapped_column(Numeric(10, 2), nullable=False)
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def __repr__(self) -> str:
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"""提供适合开发调试的对象显示。"""
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return (
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f"Book(id={self.id!r}, isbn={self.isbn!r}, "
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f"title={self.title!r}, price={self.price!r})"
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)
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def load_database_config(config_path: Path) -> dict[str, str | int]:
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"""读取并检查本地TOML数据库配置。"""
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if not config_path.exists():
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raise RuntimeError(
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"未找到 config.toml，请复制 config.example.toml 并填写练习数据库配置。"
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)
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with config_path.open("rb") as config_file:
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config_data = tomllib.load(config_file)
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database_config = config_data.get("postgresql")
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if not isinstance(database_config, dict):
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raise RuntimeError("config.toml 缺少 [postgresql] 配置节。")
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return database_config
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def create_database_url(database_config: dict[str, str | int]) -> URL:
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"""用结构化参数创建URL，避免手工拼接和处理密码特殊字符。"""
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return URL.create(
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drivername="postgresql+psycopg",
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username=str(database_config["user"]),
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password=str(database_config["password"]),
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host=str(database_config["host"]),
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port=int(database_config["port"]),
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database=str(database_config["dbname"]),
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)
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def reset_example_data(session: Session) -> None:
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"""只删除ORM-前缀的课程示例数据。"""
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session.execute(delete(Book).where(Book.isbn.like("ORM-%")))
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def add_books(session: Session) -> None:
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"""创建Python对象并交给Session持久化。"""
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books = [
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Book(
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isbn="ORM-001",
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title="Python数据库编程",
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author="小明",
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price=Decimal("68.00"),
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),
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Book(
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isbn="ORM-002",
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title="SQLAlchemy实践",
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author="小红",
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price=Decimal("88.00"),
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),
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]
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session.add_all(books)
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# flush把待处理INSERT发送到数据库，但当前事务尚未提交。
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session.flush()
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print(f"flush后第一本书ID：{books[0].id}")
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def find_books(session: Session) -> list[Book]:
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"""使用SQLAlchemy 2.x的select()查询课程图书。"""
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statement = (
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select(Book)
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.where(Book.isbn.like("ORM-%"))
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.order_by(Book.isbn)
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)
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return list(session.scalars(statement).all())
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def update_book(session: Session) -> None:
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"""查询ORM对象并修改属性，由Session跟踪变化。"""
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book = session.scalar(select(Book).where(Book.isbn == "ORM-001"))
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if book is None:
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raise RuntimeError("没有找到待修改图书ORM-001。")
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book.price = Decimal("72.00")
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def delete_book(session: Session) -> None:
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"""查询ORM对象并标记删除。"""
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book = session.scalar(select(Book).where(Book.isbn == "ORM-002"))
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if book is None:
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raise RuntimeError("没有找到待删除图书ORM-002。")
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session.delete(book)
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def demonstrate_rollback(session_factory: sessionmaker[Session]) -> None:
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"""演示异常离开Session.begin()时自动回滚。"""
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try:
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with session_factory.begin() as session:
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book = session.scalar(select(Book).where(Book.isbn == "ORM-001"))
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if book is None:
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raise RuntimeError("没有找到回滚演示图书ORM-001。")
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book.price = Decimal("1.00")
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session.flush()
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raise RuntimeError("模拟后续业务失败。")
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except RuntimeError as error:
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print(f"失败事务已回滚：{error}")
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def print_books(title: str, books: list[Book]) -> None:
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"""输出一个查询阶段的图书结果。"""
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print(title)
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for book in books:
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print(f"{book.isbn}｜{book.title}｜作者：{book.author}｜价格：{book.price}")
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def main() -> None:
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"""创建Engine和Session工厂，依次演示ORM增删改查。"""
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try:
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database_config = load_database_config(CONFIG_PATH)
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database_url = create_database_url(database_config)
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# Engine通常在应用启动时创建一次；它管理数据库方言和连接池。
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engine = create_engine(
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database_url,
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connect_args={
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"connect_timeout": int(database_config.get("connect_timeout", 10))
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},
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pool_size=5,
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max_overflow=5,
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pool_pre_ping=True,
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echo=False,
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)
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session_factory = sessionmaker(engine, expire_on_commit=False)
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# 根据模型元数据创建缺失的课程表，不会迁移已有表结构。
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Base.metadata.create_all(engine)
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with session_factory.begin() as session:
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reset_example_data(session)
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add_books(session)
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with session_factory() as session:
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print_books("新增后：", find_books(session))
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with session_factory.begin() as session:
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update_book(session)
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delete_book(session)
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with session_factory() as session:
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print_books("修改并删除后：", find_books(session))
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demonstrate_rollback(session_factory)
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with session_factory() as session:
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print_books("失败事务回滚后：", find_books(session))
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except (OSError, tomllib.TOMLDecodeError, KeyError, RuntimeError) as error:
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print(f"配置或课程数据错误：{error}")
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except SQLAlchemyError as error:
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# SQLAlchemyError是SQLAlchemy数据库访问异常的共同基础类型。
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print(f"数据库访问失败：{error}")
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if __name__ == "__main__":
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main()
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