protected function getKeysForSpecialRelation($specialType, $foreignKey = null)
    {
        switch ($specialType) {

            case static::RELATION_TYPE_FILE:
                $foreignKey   = $foreignKey ?: config('pxlcms.relations.files.keys.entry');
                $secondaryKey = config('pxlcms.relations.files.keys.field');
                break;

            case static::RELATION_TYPE_CHECKBOX:
                $foreignKey   = $foreignKey ?: config('pxlcms.relations.checkboxes.keys.entry');
                $secondaryKey = config('pxlcms.relations.checkboxes.keys.field');
                break;

            case static::RELATION_TYPE_IMAGE:
            default:
                $foreignKey   = $foreignKey ?: config('pxlcms.relations.images.keys.entry');
                $secondaryKey = config('pxlcms.relations.images.keys.field');
        }

        return [ $foreignKey, $secondaryKey ];
    }