如何在sqlalchemy中使用关系更改父级？

Question

这是与backref和delete级联关系的简单示例。

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

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('Parent.id'))
    parent = relation(
        'Parent',
        backref=backref('children', cascade='delete, delete-orphan')
    )

需要将一些子代从一个父代重新分配给另一个父代，然后需要删除另一个父代，但是sqlalchemy会在删除父代之前删除子代...

p1 = Session.query(Parent).get(...)
p2 = Session.query(Parent).get(...)

p1.children.extend(p2.children)
Session.delete(p2)
Session.commit()

日志：

... [worker 0] UPDATE child SET parent_id=%(parent_id)s WHERE child.id = %(child_id)s
...
... [worker 0] DELETE FROM child WHERE child.id = %(id)s
...
... [worker 0] DELETE FROM parent WHERE parent.id = %(id)s
...
... [worker 0] COMMIT

需要以某种方式将孩子与另一位父母分开...

Answer 1

我可以建议的唯一解决方案是：

p1 = Session.query(Parent).get(...)
p2 = Session.query(Parent).get(...)

for child in p2.children[:]:
    child.parent = p1

Session.delete(p2)
Session.commit()