def sorting(self):
        if self.qs.get('sort'):
            sorting_results = []
            for sort_field in self.qs['sort'].split(','):
                field = sort_field.replace('-', '')
                if field not in self.schema._declared_fields:
                    raise InvalidSort("{} has no attribute {}".format(self.schema.__name__, field))
                if field in get_relationships(self.schema):
                    raise InvalidSort("You can't sort on {} because it is a relationship field".format(field))
                field = get_model_field(self.schema, field)
                order = 'desc' if sort_field.startswith('-') else 'asc'
                sorting_results.append({'field': field, 'order': order})
            return sorting_results
        return []