如何在prestashop上的发票中显示wholesale_price

Question

I have a question... How to to show wholesale_price for custommer on prestashop invoice. My code in classes/product.php

$query = "SELECT wholesale_price FROM ps_product WHERE id_product =   '$id_product' ";
$result = mysql_query($query);
$row = mysql_fetch_row($result);
        if ($reduction_from_category !== false)
            $price -= $price * (float)$reduction_from_category;
            elseif ($id_group == 12)

            $price -= ($price - $row[0]);

    }

Price show in products but no in invoice. In invoice product price shows 0.00 Please help