docs: def _serialize_items(self, serializer, kind, items):
          """Return serialized items or list of ids, depending on `hydrate_XXX` query param."""
          if self.request and self.request.query_params.get('hydrate_{}'.format(kind), False):
              serializer = serializer(items, many=True, read_only=True)
              serializer.bind(kind, self)
              return serializer.data
          else:
              return [item.id for item in items]