三十八：数据库之ORM层面删除数据的注意事项

准备工作

from sqlalchemy import create_engine, Column, Integer, String, Float, Text, ForeignKeyfrom sqlalchemy.ext.declarative import declarative_basefrom sqlalchemy.orm import sessionmaker, relationship, backref# 数据库信息host = ‘127.0.0.1‘port = ‘3306‘database = ‘db_to_sqlalchemy‘username = ‘root‘password = ‘123456‘# 数据库类型+连接数据库的插件，这里使用的pymysqlDB_URI = f‘mysql+pymysql://{username}:{password}@{host}:{port}/{database}‘engine = create_engine(DB_URI)  # 创建引擎Base = declarative_base(engine)  # 使用declarative_base创建基类session = sessionmaker(engine)()

创建模型

class User(Base):    __tablename__ = ‘user‘    id = Column(Integer, primary_key=True, autoincrement=True)    username = Column(String(50), nullable=False)    def __repr__(self):        return f‘User(username: {self.username})‘class Article(Base):    __tablename__ = ‘article‘    id = Column(Integer, primary_key=True, autoincrement=True)    title = Column(String(50), nullable=False)    uid = Column(Integer, ForeignKey(‘user.id‘))    author = relationship(‘User‘, backref=‘article‘)    def __repr__(self):        return f‘Article(title: {self.title})‘Base.metadata.drop_all()  # 删除所有表Base.metadata.create_all()  # 创建表

插入数据

user = User(username=‘aaa‘)article = Article(title=‘xxxxxxx‘)article.author = usersession.add(article)session.commit()

写SQL删除user信息，报外键约束

使用sqlalchemy删除，成功

user = session.query(User).first()session.delete(user)session.commit()

可见，sqlalchemy删除时会将外键设置为null，所以为了使用外键约束，需在建表的时候，设置外键nullable=False，即不允许为空

class User(Base):    __tablename__ = ‘user‘    id = Column(Integer, primary_key=True, autoincrement=True)    username = Column(String(50), nullable=False)    def __repr__(self):        return f‘User(username: {self.username})‘class Article(Base):    __tablename__ = ‘article‘    id = Column(Integer, primary_key=True, autoincrement=True)    title = Column(String(50), nullable=False)    uid = Column(Integer, ForeignKey(‘user.id‘), nullable=False)    author = relationship(‘User‘, backref=‘article‘)    def __repr__(self):        return f‘Article(title: {self.title})‘Base.metadata.drop_all()  # 删除所有表Base.metadata.create_all()  # 创建表user = User(username=‘aaa‘)article = Article(title=‘xxxxxxx‘)article.author = usersession.add(article)session.commit()

再次删除，就会报错

user = session.query(User).first()session.delete(user)session.commit()