public function checkStockLevel($qty)
    {
        $stock_param = $this->config()->stock_param;
        $item = $this->FindStockItem();
        $stock = ($item->$stock_param) ? $item->$stock_param : 0;
        
        // if not enough stock, throw an exception
        if($stock < $qty) {
            throw new ValidationException(_t(
                "Checkout.NotEnoughStock",
                "There are not enough '{title}' in stock",
                "Message to show that an item hasn't got enough stock",
                array('title' => $item->Title)
            ));
        }
    }