WooCommerce related products – show only products from the same sub category

The issue

By default, if you have a large WooCommerce store with lots of categories and sub categories, the related products area will randomly select products from not only the same category, but any parent category above the category set for the currently displayed product. However, sometimes you may only want to show related products from the same category/sub category.

The solution

We recommend simply adding the below code to the bottom of your functions.php and job done!

NOTE: WooCommerce does cache related products for around 10 hours so you may need to test your code on a product that hasn’t been viewed.

/**
* Only show products in the same sub categories in the related products area
*
* @param $terms - Terms currently being passed through
* @param $product_id - Product ID of related products request
* @return $terms/$subcategories - Terms to be included in related products query
*/
function blz_filter_related_products_subcats_only($terms, $product_id) {
    // Check to see if this product has only one category ticked
	$prodterms = get_the_terms($product_id, 'product_cat');
	if (count($prodterms) === 1) {
		return $terms;
	}
    
    // Loop through the product categories and remove parent categories
	$subcategories = array();
	foreach ($prodterms as $k => $prodterm) {
		if ($prodterm->parent === 0) {
			unset($prodterms[$k]);
		} else {
			$subcategories[] = $prodterm->term_id;
		}
	}
	return $subcategories;
}
add_filter( 'woocommerce_get_related_product_cat_terms', 'blz_filter_related_products_subcats_only', 20, 2 );