Only shows related products from the primary category set by Yoast

woocommerce-filter-related-categories-by-taxonmy-term.php

/* Only shows related products from the primary category set by Yoast. */
function wds_display_relatd_products_from_primary_category( $related_posts, $product_id, $args  ) {

    // Get the primary category/term as saved by Yoast
    $primary_cat_id   = get_post_meta( $product_id, '_yoast_wpseo_primary_product_cat', true );
    $primary_cat_info = get_term_by( 'id', $primary_cat_id, 'product_cat' );
    $primary_cat_slug = $primary_cat_info->slug;

    // Get ids of just the posts in this category. We only want related posts from the primary category.
    $args = array(
        'status'         => 'publish',
        'category'       => array( $primary_cat_slug ),
        'return'         => 'ids',
        'excluded_ids'   => array( $product_id ),
        'posts_per_page' => -1,
        'visibility'     => 'catalog',
    );
    $related_posts = wc_get_products( $args );

    return $related_posts;

}
add_filter( 'woocommerce_related_products', 'wds_display_relatd_products_from_primary_category', 10, 3 );

Latest version of woocommerce this needed a slight adjustment:

'excluded_ids'   => array( $product_id ),

should now be

'exclude'   => array( $product_id ),

Other than that still works fine, thanks for the gist!