code: def add(self, *args, **kwargs):
          
  
          check_uniqueness = kwargs.get('check_uniqueness', True)
  
          key = self.get_storage_key(*args)
          if self.field.unique and check_uniqueness:
              self.check_uniqueness(key=key, *args)
  
          # Do index => create a key to be able to retrieve parent pk with
          # current field value]
          pk = self.instance.pk.get()
          logger.debug("adding %s to index %s" % (pk, key))
          self.connection.sadd(key, 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``