public function getSubject()
	{
		if (!$this->isNewRecord && $this->subject === null) {
			$this->subject = call_user_func([$this->model_class, 'findOne'], $this->foreign_pk);
			if ($this->subject === null) {
				$msg = Yii::t('app', 'Could not find model for attachment `{attachment}`', [
					'attachment'=>$this->id
				]);
				throw new ErrorException($msg);
			}
		}
		return $this->subject;
	}