code: def choice(self, obj):
          tree_id = getattr(obj, self.queryset.model._mptt_meta.tree_id_attr, 0)
          left = getattr(obj, self.queryset.model._mptt_meta.left_attr, 0)
          return super(MPTTModelChoiceIterator,
                       self).choice(obj) + ((tree_id, left),)