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)

def _get_simple_fields(self):
        """
        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)
        """
        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