code: def add(self, *args, **kwargs):
          
  
          check_uniqueness = kwargs.get('check_uniqueness', True)
  
          if self.field.unique and check_uniqueness:
              self.check_uniqueness(*args)
  
          key = self.get_storage_key(*args)
  
          args = list(args)
          value = args[-1]
  
          pk = self.instance.pk.get()
          logger.debug("adding %s to index %s" % (pk, key))
          self.store(key, pk, self.prepare_value_for_storage(value, pk))
          self._indexed_values.add(tuple(args))
docs: Add the instance tied to the field for the given "value" (via `args`) to the index
  
          For the parameters, see ``BaseIndex.add``
  
          Notes
          -----
          This method calls the ``store`` method that should be overridden in subclasses
          to store in the index sorted-set key