public static function getDefaultFields($model, $extra = false)
    {
        $fields = $extra ? [] : $model->attributes();
        foreach ($model->relations() as $relation) {
            $activeRelation = $model->getRelation($relation);
            if ((!$extra && $activeRelation->multiple) || ($extra && !$activeRelation->multiple)) {
                continue;
            }
            $fields[] = $relation;
        }
        return $fields;
    }