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, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from 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pymysql
DB_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 = user

session.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 = user

session.add(article)
session.commit()

再次删除，就会报错

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