MPTT tree copying branches?

Question

Django

Maybe someone faced a problem:
There is a PolymorphicMPTTModel tree. A task has appeared - if the parent of an arbitrary branch changes, then we copy the data of the branch to a new place, leaving a copy of the branch with the old id with the "deleted" object marks in the old place.
I intercept the saving of the object, look - if its parent changes - then I create a branch with the topology of the old branch, copying the data using the self.get_descendants(include_self=True) generator. And I change the parent of the original branch to a new one.
Everything is fine - only now a copy of the deleted branch - with new id - that is, the branch has been moved to a new location, and in its place there is a deleted branch with NEW id.
How to leave the OLD id of the deleted branch with the least blood, and create new ones for the moved one?

Answer 1

As I understand it, the problem you have is that you are intercepting the save when it parenthas already been changed. I would do it like this.

class MyModel(models.Model):
    def __init__(self, *args, **kwargs):
        super(MyModel, self).__init__(*args, **kwargs)
        self._initial_parent = self.parent

    def save(self, *args, **kwargs):
        new_parent = self.parent
        if new_parent != self._initial_parent:
            # Marking old branch as obsolete
            self.descendants().update(is_deleted=True)

            # Now, creating a new branch (with new IDs) at the destination node
            root = MyModel.objects.create(parent=new_parent, ...)
            [MyModel.objects.create(...) ...]

            # And cancelling the moving of the current element
            self.parent = self._initial_parent
            self.is_deleted = True

        return super(MyModel, self).save(*args, **kwargs)