docs: def _closure_parent_pk(self):
          """What our parent pk is in the closure tree."""
          if hasattr(self, "%s_id" % self._closure_parent_attr):
              return getattr(self, "%s_id" % self._closure_parent_attr)
          else:
              parent = getattr(self, self._closure_parent_attr)
              return parent.pk if parent else None