/**
  * Marks this collection as changed/dirty.
  */
 private function changed()
 {
     if (!$this->isDirty) {
         $this->isDirty = true;
         if ($this->dm && $this->mapping !== null && $this->mapping['isOwningSide'] && $this->dm->getClassMetadata(get_class($this->owner))->isChangeTrackingNotify()) {
             $this->uow->scheduleForDirtyCheck($this->owner);
         }
     }
 }