.relationship(…,cascade="all") 当使用 session.execute(delete(Parent))“批量”删除多个父级时，不会删除子级

Question

我用 Python 编写了一些代码来管理一些数据库内容。 我想做的就是删除一对多关系的所有父母，这应该自动删除所有孩子。这是我的示例代码：

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, Session
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    single_parent = True
    children = relationship("Child", back_populates="parent", cascade="all, delete-orphan")

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    parent_id = Column(Integer, ForeignKey('parents.id'))
    parent = relationship('Parent', back_populates='children')



engine = create_engine("sqlite:///test.db", echo=True)
Base.metadata.create_all(bind=engine)


session = Session(engine)
# create data
parent1 = Parent(name='Parent 1', children=[Child(name='Child 1'), Child(name='Child 2')])
parent2 = Parent(name='Parent 2', children=[Child(name='Child 3'), Child(name='Child 4')])
session.add_all([parent1, parent2])
session.commit()

# Check
print("Before:")
print(session.query(Parent).all())
print(session.query(Child).all())

# Delete
session.query(Parent).delete()
session.commit()

运行时，父元素会被删除，但子元素不会被删除。 我在这里做错了什么？ 非常感谢您的帮助！

Answer 1

您需要修改

parent_id

声明：

更换：

parent_id = Column(Integer, ForeignKey('parents.id'))

与：

parent_id = Column(Integer, ForeignKey('parents.id', ondelete='CASCADE'))

但是，当您使用

SQLite

作为数据库时，这是不够的，因为 默认情况下未启用外键约束。 您可以在数据库级别使用

PRAGMA

指令启用外键约束：（请参阅下面 @GordThompson 的评论）

PRAGMA foreign_keys = ON;

或者您可以通过监听事件使用“SQLAlchemy”来处理此功能：

from sqlalchemy.event import listens_for
from sqlalchemy.engine import Engine
from sqlite3 import Connection as SQLite3Connection

# enable foreign keys. if you do not do this, ON DELETE CASCADE fails silently!
@listens_for(Engine, 'connect')
def _set_sqlite_pragma(dbapi_connection, connection_record):
    if isinstance(dbapi_connection, SQLite3Connection):
        cursor = dbapi_connection.cursor()
        cursor.execute('PRAGMA foreign_keys=ON;')
        cursor.close()

测试：

# Check
print("Before:")
print(session.query(Parent).all())
print(session.query(Child).all())

# Delete only one parent
session.query(Parent).filter_by(name='Parent 2').delete()
session.commit()

# Check
print("After:")
print(session.query(Parent).all())
print(session.query(Child).all())

输出：

# I just modified the __repr__ of each table

Before:
[Parent(id=1, name='Parent 1'), Parent(id=2, name='Parent 2')]
[Child(id=1, name='Child 1', parent_name='Parent 1'), Child(id=2, name='Child 2', parent_name='Parent 1'), Child(id=3, name='Child 3', parent_name='Parent 2'), Child(id=4, name='Child 4', parent_name='Parent 2')]

After:
[Parent(id=1, name='Parent 1')]
[Child(id=1, name='Child 1', parent_name='Parent 1'), Child(id=2, name='Child 2', parent_name='Parent 1')]

Answer 2

您不需要分别删除父级和子级，如果删除父级，子级也会被删除。

此代码将起作用：

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, Session
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    single_parent = True
    children = relationship("Child", back_populates="parent", cascade="all, delete-orphan")

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    parent_id = Column(Integer, ForeignKey('parents.id'))
    parent = relationship('Parent', back_populates='children')

engine = create_engine("sqlite:///test.db", echo=True)
Base.metadata.create_all(bind=engine)

session = Session(engine)

# create data
parent1 = Parent(name='Parent 1', children=[Child(name='Child 1'), Child(name='Child 2')])
parent2 = Parent(name='Parent 2', children=[Child(name='Child 3'), Child(name='Child 4')])
session.add_all([parent1, parent2])
session.commit()

# Check
print("Before:")
print(session.query(Parent).all())
print(session.query(Child).all())

# Delete
session.query(Parent).delete()
session.commit()