Manually changing the parent of a child model is silently failing.

[Keith Hostetler]

To be honest, I have no idea if it is supposed to be allowed to set the parent via the parent_ptr, but it isn't working. If it isn't allowed, it should be reporting an error.

Example:

class Parent(models.Model):
    field1 = models.TextField(default="abc")

class Child(Parent):
    field2 = models.TextField(default="abc")

def change_parent():
    child = Child.objects.get(id=1)
    new_parent = Parent.objects.create()

    print child.id  # Prints 1
    print child.parent_ptr.id  # Prints 1
    print new_parent.id  # We'll say it is number 5
    child.parent_ptr = new_parent

    print child.id  # Prints 1
    print child.parent_ptr.id  # Prints 5
    child.save()

    print child.id  # Prints 1
    print child.parent_ptr.id  # Prints 1

This applies to the update function as well. I tried:

    Child.objects.filter(id=1).update(parent_ptr=new_parent)
    child = Child.objects.get(id=1)

    print child.id  # Prints 1
    print child.parent_ptr.id  # Prints 1

[Tim Graham]

The first point seems to be a valid issue. In particular, after assigning a new parent, the pk attribute is updated, but not the id. I think this can be fixed but I'm not 100% sure.

>>> child.parent_ptr = new_parent
>>> child.pk
2
>>> child.id
1

As for the second issue, QuerySet.update() is working for me.

>>> child = Child.objects.create()
>>> child_pk = child.id
>>> new_parent = Parent.objects.create()
>>> Child.objects.filter(id=child.pk).update(parent_ptr=new_parent)
>>> Child.objects.get(pk=child_pk)
...
DoesNotExist: Child matching query does not exist.

(make sense since the Child with the old pk no longer exists)

[Keith Hostetler]

Replying to timgraham:

As for the second issue, QuerySet.update() is working for me.

>>> child = Child.objects.create()
>>> child_pk = child.id
>>> new_parent = Parent.objects.create()
>>> Child.objects.filter(id=child.pk).update(parent_ptr=new_parent)
>>> Child.objects.get(pk=child_pk)
...
DoesNotExist: Child matching query does not exist.

(make sense since the Child with the old pk no longer exists)

Ah, yes. You are right. I didn't technically test that code, but was written based upon what I actually did and I realize now that I simply had a reference issue.

[Paulo]

PR ​https://github.com/django/django/pull/8626

[Tim Graham <timograham@…>]

In c4329271:

Fixed #26362 -- Fixed update of the inherited id field of an object when its parent changes.