public static function isReferenced($id)
 {
     require_once 'Training.php';
     $training = new Training();
     $select = $training->select();
     $select->where("training_location_id = ?", $id);
     $select->where("is_deleted = 0");
     if ($training->fetchRow($select)) {
         return true;
     }
     return false;
 }