Get page IDs from nav items

Question

I am trying to get all the IDs in my navigation and convert it to a string so that I can use it with wp_query to get the pages that are only listed in the nav. The code to get the IDs:

$menu_name = 'primary';

if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $menu_name ] )) {
    $menu = wp_get_nav_menu_object( $locations[ $menu_name ] );
    $menu_items = wp_get_nav_menu_items($menu->term_id);
}

foreach ( $menu_items as $menu_item ) {
    $numbers[] = $menu_item->ID;
}

$number = implode(',',$numbers);

But it is returning the wrong numbers. When I go to the admin area and hover my mouse over the pages, the number that shows up in the address bar on the bottom of the screen doesn't match. What am I doing wrong?

Answer 1

Menu items are stored in the posts table with a post_type of nav_menu_item. So, what you are returning is the ID of the menu item itself, not what it points to.

The page/post ID that the menu item refers to is stored in the postmeta table, with a post_id that matches the menu item ID and meta_key = _menu_item_object_id. The easiest way to get the meta_value (ie the page being pointed to by the menu item) is to use something like this:

$numbers[] = get_post_meta( $menu_item->ID, '_menu_item_object_id', true );

Answer 2

I wouldn't be able to tell you when this change happened but as of WordPress version 5.2.4 you can use object and object_id to retrieve the page ID. Here's how I would do this:

/**
 * The `0` is added as a default case.
 * In case our menu does not contain any pages this
 * will prevent our query from returning all pages
 */
$page_ids           = array( 0 );
$registered_menu    = 'my_registered_menu';
$locations          = get_nav_menu_locations();

if( ! empty( $locations ) && isset( $locations[ $registered_menu ] ) ) {

    $menu = wp_get_nav_menu_object( $locations[ $registered_menu ] );
    $menu_items = wp_get_nav_menu_items( $menu->term_id );

    if( ! empty( $menu_items ) ) {

        foreach( $menu_items as $item ) {

            // Only grab page IDs
            if( 'page' !== $item->object ) {
                continue;
            }

            $page_ids[] = $item->object_id;

        }

    }

}

$page_query = new WP_Query( array(
    'post_type'     => 'page',
    'post_status'   => 'publish',
    'no_found_rows' => true,
    'post__in'      => $page_ids,
    'posts_per_page'=> 100, // Large upper limit
) );