docs: def _add_to_index(self, att, val=None, pipeline=None):
          """
          Adds the id to the index.
  
          This also adds to the _indices set of the object.
          """
          index = self._index_key_for(att)
          if index is None:
              return
          t, index = index
          if t == 'attribute':
              pipeline.sadd(index, self.id)
              pipeline.sadd(self.key()['_indices'], index)
          elif t == 'list':
              for i in index:
                  pipeline.sadd(i, self.id)
                  pipeline.sadd(self.key()['_indices'], i)
          elif t == 'sortedset':
              zindex, index = index
              pipeline.sadd(index, self.id)
              pipeline.sadd(self.key()['_indices'], index)
              descriptor = self.attributes[att]
              score = descriptor.typecast_for_storage(getattr(self, att))
              pipeline.zadd(zindex, self.id, score)
              pipeline.sadd(self.key()['_zindices'], zindex)