Required functionality.

def find_by_index(self, cls, index_name, value):
        """Required functionality."""
        query_args = {
            index_name + '__eq': DynamoMappings.map_index_val(value),
            'index': gsi_name(index_name)
        }

        final_results = []
        for db_result in self.get_class_table(cls).query_2(**query_args):
            obj = cls.from_data(db_result['value'])
            final_results.append(obj)

        return final_results