如何按价格按类别对产品进行分类，但仅限于OpenCart中的库存？

How to sort products in category by price but out of stock be on the end on category? I want to show first stock items sorted by price.

I edit catalog/controller/product/category.php but I can't make quantity argument.

  if (isset($this->request->get['sort'])) {
     $sort = $this->request->get['sort'];
  } else {
     //$sort = 'p.date_added';
     $sort = 'p.price';
  }

  if (isset($this->request->get['order'])) {
     $order = $this->request->get['order'];
  } else {
     //$order = 'DESC';
     $order = 'ASC';
  }

Opencart 1.5.4