/**
  * return foreign keys and entity references
  *
  * @static
  * @access public
  * @return array of CRM_Core_EntityReference
  */
 static function getReferenceColumns()
 {
     if (!self::$_links) {
         self::$_links = array(new CRM_Core_EntityReference(self::getTableName(), 'contact_id', 'civicrm_contact', 'id'));
     }
     return self::$_links;
 }