Getting django-mptt to play nice with django-reversion

Question

I'm working on a project that is using django-reversion to track changes and give the ability to revert to earlier states, and django-mptt for some tree-shaped models. The delete and revert functionality, however, have some funky behavior. I'm working with demo data that looks something like this:

Big Company
    Sub Company 1
    Sub Company 2
      Tiny Company 1
      Tiny Company 2

I've rigged django-reversion so that deleting a node also saves all its sub-nodes to the revision -- so, deleting Big Company deletes the whole tree, and then reverting it (or really, any of the sub nodes) also reverts the whole tree. So far, so good.

Things start getting weird, however, when I try to delete/revert sub-nodes. For example, deleting 'Sub company 1' causes 'Sub Company 2' and it's descendants to stop rendering in the template (although this behavior is, bizarrely, somewhat inconsistent). Or, should I use django-reversion to restore 'Sub Company 2' after a delete, neither the 'Tiny Companies' will render in the template (although quickly ducking into shell and calling the objects shows that they are still in the database, and that they still have 'Sub Company 2' as their parent.

All of these problems can be fixed by calling Company.objects.rebuild(), which restores the tree to its proper representation -- but the production ready version of this project could have a lot of data in the database, and since that's a whole-table activity it would be prohibitively expensive. Any ideas as to what I could do to fix this issue?

Answer 1

Well, I found a way to get this to work. Instead of simply deleting the MPTT node, I move it so that it's in a new root position. This reorganizes the tree properly in a way that simply deleting the node doesn't seem to. Of course, I'd like to be able to reattach the node after, so I store the parents primary key in a supplemental revision metadata option. Relevant code looks like this:

class MPTT_Parent(models.Model):
    revision = models.OneToOneField(Revision)
    parent_id = models.IntegerField()

def remove_company(obj):
    with transaction.atomic() and reversion.create_revision():

         #These lines are to preserve the integrity of the tree before deleting it
         #Objects deleted from django-mptt trees don't automatically update the positions
        if not obj.is_root_node():
            reversion.add_meta(MPTT_Parent,parent_id=obj.parent.id)
        obj.move_to(target=None,position='first-child')

        obj.save()

        #Save all associated descendant information into the revision
        for child in obj.get_descendants(include_self=False):
            child.save()

   obj.delete()

def restore_company(version):
    #get the parent id.  If the parent has been deleted or was never set, 
    #keeps the node as root
    company = revert_object(version) #custom function that reverts given object
    try:
         parent = Company.objects.get(id=version.revision.mptt_parent.parent_id)
    except (ObjectDoesNotExist, AttributeError):
         pass
    else:
         company.move_to(target=parent,position='first-child')

    return company