Срабатывание плагина, если tv изменился

<?php
/*
*  Action: OnDocFormRender, OnDocFormSave 
*/
global $modx;

if ($mode == modSystemEvent::MODE_UPD ) {
	$parent = $resource->get('parent');
	$isDiscountChanged = 0;
	
	if ($parent == "9") {
		$startTime = microtime(true);
		$brand = $resource->get('menutitle');

		switch ($modx->event->name) {
			case 'OnDocFormRender':
				$_SESSION['discount_before'] = (float)$resource->getTVValue(13); // скидка при загрузке

				$modx->log(modX::LOG_LEVEL_ERROR, '$discount_before: ' . $_SESSION['discount_before']);
			break;
			case 'OnDocFormSave':
				$discount = (float)$resource->getTVValue(13); // скидка при сохранении
				$priceCoeff = (float)$resource->getTVValue(14); // коэфф. при сохранении

				if($discount != $_SESSION['discount_before']){
					$isDiscountChanged = 1;
				}
				
				if ($isDiscountChanged) {
					
					{...} 	// получение выборки из базы

					if ( $query -> prepare() && $query -> stmt -> execute() ) {
						while ( $queryRow = $query -> stmt -> fetch( PDO::FETCH_ASSOC ) ) {
							$thisProject = $modx -> getObject( 'modResource', $queryRow['id'] );
							$price = $thisProject -> getTVValue( 1 );

							$discountPrice = round((1-$discount/100)*$price);
							$thisProject->setTVValue(2, $discountPrice); 	//внесение цены со скидкой	
					}
				}

				unset($_SESSION['discount_before']);


				$finishTime = microtime(true);
				$timeScript = $finishTime - $startTime;	
				$modx->log(modX::LOG_LEVEL_ERROR, 'setDiscount checked ' . $i . ' elements');
				$modx->log(modX::LOG_LEVEL_ERROR, 'Time wasted: ' . round($timeScript,4) . 'sec');

			break;
		}
	}
}