docs: def hook_get_queryset(self, changelist, request):
          """Triggered by `ChangeList.get_queryset()`."""
          changelist.check_field_exists(self.left_field)
          changelist.check_field_exists(self.right_field)
          self.pid = self.get_pid_from_request(changelist, request)
  
          # Get parent item first.
          qs = changelist.root_queryset
  
          if self.pid:
              self.parent = qs.get(pk=self.pid)
              changelist.params.update(self.get_immediate_children_filter(self.parent))
  
          else:
              changelist.params[self.level_field] = self.root_level
              self.parent = qs.get(**{key: val for key, val in changelist.params.items() if not key.startswith('_')})