/**
  * Returns foreign keys and entity references
  *
  * @return array
  *   [CRM_Core_Reference_Interface]
  */
 static function getReferenceColumns()
 {
     if (!self::$_links) {
         self::$_links = static::createReferenceColumns(__CLASS__);
         self::$_links[] = new CRM_Core_Reference_Basic(self::getTableName(), 'domain_id', 'civicrm_domain', 'id');
         self::$_links[] = new CRM_Core_Reference_Basic(self::getTableName(), 'parent_id', 'civicrm_navigation', 'id');
     }
     return self::$_links;
 }