/**
  * return foreign keys and entity references
  *
  * @static
  * @access public
  * @return array of CRM_Core_Reference_Interface
  */
 static function getReferenceColumns()
 {
     if (!self::$_links) {
         self::$_links = static::createReferenceColumns(__CLASS__);
         self::$_links[] = new CRM_Core_Reference_Basic(self::getTableName(), 'mailing_queue_id', 'civicrm_mailing_event_queue', 'id');
         self::$_links[] = new CRM_Core_Reference_Basic(self::getTableName(), 'activity_id', 'civicrm_activity', 'id');
     }
     return self::$_links;
 }