docs: def get_descendants(self, include_self=False, depth=None):
          """Return all the descendants of this object."""
          params = {"%s__parent" % self._closure_childref():self.pk}
          if depth is not None:
              params["%s__depth__lte" % self._closure_childref()] = depth
          descendants = self._toplevel().objects.filter(**params)
          if not include_self:
              descendants = descendants.exclude(pk=self.pk)
          return descendants.order_by("%s__depth" % self._closure_childref())