Prepare "sort" options to use when calling the collection, depending
        on "_sort" and "_sort_limits" attributes

def _prepare_sort_options(self, has_pk):
        """
        Prepare "sort" options to use when calling the collection, depending
        on "_sort" and "_sort_limits" attributes
        """
        sort_options = {}
        if self._sort is not None and not has_pk:
            sort_options.update(self._sort)
        if self._sort_limits is not None:
            if 'start' in self._sort_limits and 'num' not in self._sort_limits:
                self._sort_limits['num'] = -1
            elif 'num' in self._sort_limits and 'start' not in self._sort_limits:
                self._sort_limits['start'] = 0
            sort_options.update(self._sort_limits)
        if not sort_options and self._sort is None:
            sort_options = None
        return sort_options