Add custom link on the fly to nav menu

Question

I need to add custom links, on the fly, to the navigation menu. I can add custom links to the first level of items (created via Appearance > Menus), but for some reason, I cannot add a custom link, child of another custom link create previously..

Here's my code:

function on_the_fly($items) {

    $menu_items = array();

    foreach ($items as $item) {
        $item->menu_order = count($menu_items) + 1;
        $menu_items[] = $item;

        $new_menu_item = new \WP_Post((object) array(
            'ID' => $item->ID . "00",
        ));

        $new_menu_item->post_author = 1;
        $new_menu_item->comment_status = 'closed';
        $new_menu_item->ping_status = 'closed';
        $new_menu_item->post_type = 'nav_menu_item';
        $new_menu_item->post_title = 'test 1';
        $new_menu_item->post_name = 'test 1';
        $new_menu_item->filter = 'raw';
        $new_menu_item->object_id = $new_menu_item->ID;
        $new_menu_item->object = 'custom';
        $new_menu_item->type = 'custom';
        $new_menu_item->type_label = 'Custom Link';
        $new_menu_item->menu_order = count($menu_items) + 1;
        $new_menu_item->menu_item_parent = $item->ID;
        $new_menu_item->url = str_replace("//", "/", $item->url . "/" . sanitize_title('test 1'));
        $new_menu_item->title = 'test 1';

        $menu_items[] = $new_menu_item;

        $new_menu_item_2 = new \WP_Post((object) array(
            'ID' => $new_menu_item->ID . "00",
        ));

        $new_menu_item_2->post_author = 1;
        $new_menu_item_2->comment_status = 'closed';
        $new_menu_item_2->ping_status = 'closed';
        $new_menu_item_2->post_type = 'nav_menu_item';
        $new_menu_item_2->post_title = 'test 2';
        $new_menu_item_2->post_name = 'test 2';
        $new_menu_item_2->filter = 'raw';
        $new_menu_item_2->object_id = $new_menu_item->ID;
        $new_menu_item_2->object = 'custom';
        $new_menu_item_2->type = 'custom';
        $new_menu_item_2->type_label = 'Custom Link';
        $new_menu_item_2->menu_order = count($menu_items) + 1;
        $new_menu_item_2->menu_item_parent = $new_menu_item->ID;
        $new_menu_item_2->url = str_replace("//", "/", $item->url . "/" . sanitize_title('test 2'));
        $new_menu_item_2->title = 'test 2';

        $menu_items[] = $new_menu_item_2;

    }
    return $menu_items;
}

add_filter('wp_nav_menu_objects', 'on_the_fly', 1,1);

Result:

> - Application
>  - test 1
> - test 2

And not:

> - Application
>   - test 1
>     - test 2

How can I achieve the desired result?

Answer 1

There is a really good example on this page if you want to have a look and see if it helps - https://isabelcastillo.com/dynamically-sub-menu-item-wp_nav_menu

Answer 2

It seems that the missing argument was "db_id". If defined, the tree will be created as intended.

$new_menu_item->db_id = $new_menu_item->ID;

Thanks