docs: def save(self, *args, **kwargs):
          """
          Set the initial ordering value.
          """
          if self._order is None:
              lookup = self.with_respect_to()
              lookup["_order__isnull"] = False
              concrete_model = base_concrete_model(Orderable, self)
              self._order = concrete_model.objects.filter(**lookup).count()
          super(Orderable, self).save(*args, **kwargs)