Thirty-eight: ORM-level database of the Notice on deleting data

Ready to work

Import create_engine SQLAlchemy from, the Column, Integer, String, the Float, the Text, a ForeignKey 
from sqlalchemy.ext.declarative Import declarative_base 
from sqlalchemy.orm Import sessionmaker, Relationship, backref 

# database information 
Host = '127.0.0.1' 
Port = '3306' 
Database = 'db_to_sqlalchemy' 
username = 'the root' 
password = '123456' 

widget database # + type connection to the database, pymysql used herein 
DB_URI = f'mysql + pymysql: // { username}: {password} @ {host}: { } Port / Database {} ' 

engine = create_engine (DB_URI) engine # Create 
base = declarative_base (engine) # create a base class used declarative_base 
session = sessionmaker (engine) ()

Create a model

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

Insert data

user = User(username='aaa')
article = Article(title='xxxxxxx')
article.author = user

session.add(article)
session.commit()

Write SQL delete user information, reported foreign key constraint

Use sqlalchemy deleted successfully

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

Visible, sqlalchemy time will remove the foreign key is set to null, so in order to use the foreign key constraint, the need to build the table, the foreign key nullable = False, which does not allow empty

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()

Thirty-eight: ORM-level database of the Notice on deleting data

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