我可能在SQLAlchemy的级联选项上漏了一些微不足道的东西,因为我无法使一个简单的级联删除操作正确运行 - 如果删除一个父元素,则子元素会持久化,但外键变为null
。
我在这里放了一个简明的测试案例:
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
Parent和Child之间存在一个简单的一对多关系。该脚本创建了一个父对象,并添加了三个子对象,然后提交了事务。接下来,它删除了父对象,但是这些子对象仍然存在。为什么会出现这种情况?如何使孩子对象进行级联删除?
session.delete(parent)
后面加上session.commit()
是不必要的。 - NMO