Extract the actual sort key (not pattern) from operations.

def actual_sort(self):
        """Extract the actual sort key (not pattern) from operations."""
        if not self._actual_sort:

            # trigger evaluation of operation
            if self.operation in ['query', 'getmore']:
                self._actual_sort = self._find_pattern('orderby: ',
                                                        actual=True)

        return self._actual_sort