Database SQLAlchemy：级联删除

我肯定遗漏了SQLAlChemy的级联选项中一些微不足道的东西，因为我不能让简单的级联删除来正确操作--如果父元素是DELETED元素，则子元素将使用null个外键保持不变.

我在这里放了一个简明的测试用例:

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Parent(Base):
    __tablename__ = "parent"
    id = Column(Integer, primary_key = True)

class Child(Base):
    __tablename__ = "child"
    id = Column(Integer, primary_key = True)
    parentid = Column(Integer, ForeignKey(Parent.id))
    parent = relationship(Parent, cascade = "all,delete", backref = "children")

engine = create_engine("sqlite:///:memory:")
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)

session = Session()

parent = Parent()
parent.children.append(Child())
parent.children.append(Child())
parent.children.append(Child())

session.add(parent)
session.commit()

print "Before delete, children = {0}".format(session.query(Child).count())
print "Before delete, parent = {0}".format(session.query(Parent).count())

session.delete(parent)
session.commit()

print "After delete, children = {0}".format(session.query(Child).count())
print "After delete parent = {0}".format(session.query(Parent).count())

session.close()

输出:

Before delete, children = 3
Before delete, parent = 1
After delete, children = 3
After delete parent = 0

父母和子元素之间有一种简单的一对多关系.脚本创建一个父级，添加3个子级，然后提交.接下来，它会删除父对象，但子对象会保留.为什么？我该如何让子元素们go 做？