protected static function executeFieldsSerially(ExecutionContext $context, ObjectType $parent, $source, $fields)
    {
        $results = [];
        foreach ($fields as $response => $asts) {
            $result = self::resolveField($context, $parent, $source, $asts);

            if ($result !== self::$UNDEFINED) {
                // Undefined means that field is not defined in schema.
                $results[$response] = $result;
            }
        }

        return $results;
    }