code: def _get_simple_fields(self):
          
          fields = []
          for field_name in self.cls._fields:
              field = self.cls.get_field(field_name)
              if not isinstance(field, MultiValuesField):
                  fields.append(field_name)
          return fields
docs: Return a list of the names of all fields that handle simple values
          (StringField or InstanceHashField), that redis can use to return values via
          the sort command (so, exclude all fields based on MultiValuesField)