Prepare sort options for _values attributes.
        If we manager sort by score after getting the result, we do not want to
        get values from the first sort call, but only from the last one, after
        converting results in zset into keys

def _prepare_sort_options(self, has_pk):
        """
        Prepare sort options for _values attributes.
        If we manager sort by score after getting the result, we do not want to
        get values from the first sort call, but only from the last one, after
        converting results in zset into keys
        """
        sort_options = super(ExtendedCollectionManager, self)._prepare_sort_options(has_pk)

        if self._values:
            # if we asked for values, we have to use the redis 'sort'
            # command, which is able to return other fields.
            if not sort_options:
                sort_options = {}
            sort_options['get'] = self._values['fields']['keys']

        if self._sort_by_sortedset_after:
            for key in ('get', 'store'):
                if key in self._sort_by_sortedset:
                    del self._sort_by_sortedset[key]
            if sort_options and (not has_pk or self._want_score_value):
                for key in ('get', 'store'):
                    if key in sort_options:
                        self._sort_by_sortedset[key] = sort_options.pop(key)
            if not sort_options:
                sort_options = None
        return sort_options