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

def add(self, *args, **kwargs):
        """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

        """

        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))