I register a taxonomy using standard actions:

add_action( 'init', 'product_brand_order_taxonomy' );
function product_brand_order_taxonomy()  {
    $labels = array(
        'name'                       => 'Brand Heirarchy',
        'singular_name'              => 'Brand Heirarchy',
        'menu_name'                  => 'Brand Heirarchy',
        'all_items'                  => 'All Brand Heirarchies',
        'parent_item'                => 'Parent Brand Heirarchy',
        'parent_item_colon'          => 'Parent Brand Heirarchy:',
        'new_item_name'              => 'New Brand Heirarchy Name',
        'add_new_item'               => 'Add New Brand Heirarchy',
        'edit_item'                  => 'Edit Brand Heirarchy',
        'update_item'                => 'Update Brand Heirarchy',
        'separate_items_with_commas' => 'Separate Brand Heirarchy with commas',
        'search_items'               => 'Search Brand Heirarchies',
        'add_or_remove_items'        => 'Add or remove Brand Heirarchies',
        'choose_from_most_used'      => 'Choose from the most used Brand Heirarchies',
    );
    $args = array(
        'labels'                     => $labels,
        'hierarchical'               => true,
        'public'                     => true,
        'show_ui'                    => true,
        'show_admin_column'          => true,
        'show_in_nav_menus'          => true,
        'show_tagcloud'              => true,
    );
    register_taxonomy( 'brand_heirarchy', 'product', $args );
    register_taxonomy_for_object_type( 'brand_heirarchy', 'product' );
}

Later, I want to get the product based on some parameters. I also want the brand hierarchy (brand_heirarchy) to be included in the results.

$products = array(
    'post_status' => 'publish',
    'limit' => -1,
    'category' => $parent_brand_slugs
);

$products = wc_get_products($product_args);

In the returned data, it will give me the category ID, for example:

[category_ids] => Array
    (
        [0] => 30
        [1] => 27
        [2] => 25
        [3] => 24
    )

But I want it to also return the brand-level ID associated with the product.

Most of the content I see is about how to filter products by custom taxonomies, I just want to know which taxonomies are associated with products.

I found out that Woocommerce doesn't allow this: https://github.com/woocommerce/woocommerce/issues/13138

I tried using the filter woocommerce_product_object_query_args, but couldn't figure out how to use it. There is also a product search filter, but it doesn't seem to apply.

Currently, I think the best way is to loop through each product and use something like wp_get_object_terms($data['id'], 'brand_heirarchy'), but this doesn't seem efficient.

I haven't used WordPress in a long time, so I'd really like to know if there's a more efficient way to get the taxonomies associated with each product.

Thanks.

Oh, wp_get_object_terms and wp_get_post_terms seem to return empty arrays, while products are indeed assigned brand tiers.