Have WooCommerce use only the subcategory a product is in for related products (not parent category)

related-products.php

// Stop related posts using tags
//add_filter('woocommerce_product_related_posts_relate_by_tag', function() {
//    return false;
//});

add_filter('woocommerce_related_products_args', 'filter_woocommerce_related_products_args');

function filter_woocommerce_related_products_args($args)
{
    global $post;

    // get the cats this product is in
    $terms = get_the_terms($post->ID, 'product_cat');

    // if there is only one category jump out.
    if (count($terms) === 1) {
        return $args;
    }

    $cats = array();
    // remove anything that is a parent cat
    foreach ($terms as $k => $term) {
        if ($term->parent === 0) {
            unset($terms[$k]);
        } else {
            // build list of terms we do want (children)
            $cats[] = $term->term_id;
        }
    }

    $post_ids = get_posts(array(
        'post_type' => 'product',
        'numberposts' => -1, // get all posts.
        'tax_query' => array(
            array(
                'taxonomy' => 'product_cat',
                'field' => 'term_id',
                'terms' => $cats,
            ),
            // Remove current product
            array(
                'taxonomy' => 'product_cat',
                'field' => 'term_id',
                'terms' => $post->ID,
                'operator' => 'NOT IN',
            ),
        ),
        'fields' => 'ids', // Only get post IDs
    ));

    // Alter the query
    $args['post__in'] = $post_ids;

    return $args;
}

Here's my solution for newer WooCommerce, add this to functions.php and include this filter in post query in carousel or somewhere :p merry xmas!

/**
 * GET RELATED PRODUCTS FROM DIRECT CATEGORY
 */
add_filter( 'woocommerce_product_related_posts', 'woocommerce_get_direct_related_products' );
function woocommerce_get_direct_related_products($args) {
    global $woocommerce, $product;

    // Related products are found from category
    $cats_array = array(0);

    // Get categories
    $terms = wp_get_post_terms( $product->id, 'product_cat' );

    //Select only the category which doesn't have any children
    foreach ( $terms as $term ) {
        $children = get_term_children( $term->term_id, 'product_cat' );
        if ( !sizeof( $children ) )
            $cats_array[] = $term->term_id;
    }

    // Don't bother if none are set
    if ( sizeof( $cats_array ) == 1 ) return $args;


    // Meta query
    $meta_query = array();
    $meta_query[] = $woocommerce->query->visibility_meta_query();
    $meta_query[] = $woocommerce->query->stock_status_meta_query();

    $limit = -1;

    $post_ids = get_posts(array(
        'orderby'       => 'rand',
        'posts_per_page'=> $limit,
        'post_type'     => 'product',
        'fields'        => 'ids',
        'meta_query'    => $meta_query,
        'post__not_in' => array( $product->get_id() ),  //this is the way i remove current prod xd
        'tax_query' => array(
            array(
                'taxonomy'  => 'product_cat',
                'field'     => 'id',
                'terms'     => $cats_array
            ),
        ),
        'fields' => 'ids', // Only get post IDs
    ));

    // Alter the query
    $args['post__in'] = $post_ids;

    return $args;

}

Thanks @m16averick.

@m16averick - It's not working for me, but I have hierarchal categories. Can it be made to work that way or with tags?

I tested the code on my site.
I see no effect on how it works in related products. Probably because all the products also have the "Uncategorized" category. Should the code be changed to remove this category from the search for related products?

I'm using the DIVI theme, I don't wish it was a DIVI problem. What do you think?

Thanks

I find this plugin which is suitable for our requirement - Related Products for WooCommerce by WebToffee,
After installing the plugin you can able to display products based on category and tags. If you want to display products based on sub-category follow this tutorial. https://www.webtoffee.com/related-products-woocommerce-user-guide/#sub_category

I find this plugin which is suitable for our requirement - Related Products for WooCommerce by WebToffee, After installing the plugin you can able to display products based on category and tags. If you want to display products based on sub-category follow this tutorial. https://www.webtoffee.com/related-products-woocommerce-user-guide/#sub_category

Thanks alot