How to determine product's category in Search's resoults?

How to determine product's category in Search's resoults?

Dear Team,

I would like to enable buy button only for the products in specified categories.
Actually I could solve this in List view and Product Page view. It is not working only when I saerch for the items where I would like to show buy button.

I used these codes in Template directory's list_products folder in product.php file for example:
$category_id = JRequest::getInt('category_id');
<?php } if ($product->buy_link && $category_id == 979){?>
<a class="btn btn-small button_buy" href="<?php print $product->buy_link?>"><?php print _JSHOP_BUY?></a> &nbsp;
<?php }?>

Why it is not working when I make a search?

---

Aw: How to determine product's category in Search's resoults?

For order development use kontakt.