Highlight the CPT (Custom Post Type) Archive & its Parents ("current-menu-parent") & its Ancestors ("current-menu-ancestor") in the WordPress Navigation wp_nav_menu() when viewing the Archive itself or a single detail page of the CPT. Let's say you have CPT called "projects" and you have a WordPress menu set as "My Work (link to a page) > Projec…

Highlight CPT Archive and wp_nav_menu parents

/**
 * Gist Name: Highlight CPT Archive and wp_nav_menu parents
 * Author: VlooMan
 * Author URI: http://ishyoboy.com
 *
 * Highlight the CPT (Custom Post Type) Archive & its Parents ("current-menu-parent")
 * & its Ancestors ("current-menu-ancestor") in the WordPress Navigation wp_nav_menu() when viewing the Archive itself
 * or a single detail page of the CPT.
 *
 * Let's say you have CPT called "projects" and you have a WordPress menu set as
 * "My Work (link to a page) > Projects (CPT Archive)". If you visit the CPT Archive the "My Work" item would not be
 * originally highlighted, only the "Projects" would be. If you visit the detail of a given project nothing would be
 * highlighted in the navigation (neither "My Work" nor "Projects"). The code below solves the problem.
 *
 * The code uses a WordPress hook and 99% of the code is copied from the original WordPress function which for some
 * reason does not do this highlighting. Please let me know if the code helped in the comments below.
 *
 * Simply copy this code to your theme's or child-theme's functions.php file.
 *
 * P.S. Just make sure to add the nav menu item as "CPT Archive" and not a "Custom Link".
 *
 * @param array $menu_items - All Navigation Menu Items
 * @param array $args - some arguments we do not use right now
 *
 * @return array $menu_items
 */

function my_highlight_cpt_archive_and_ancestors_in_menu( $menu_items, $args ) {
	global $wp_query, $wp_rewrite;

	$queried_object = $wp_query->get_queried_object();

	$active_object = '';
	$active_ancestor_item_ids = array();
	$active_parent_item_ids = array();
	$active_parent_object_ids = array();
	$possible_taxonomy_ancestors = array();

	// Highligh the current post's archive & prepare parents for highlighting
	foreach ( (array) $menu_items as $key => $menu_item ) {

		if (
			( 'post_type_archive' == $menu_item->type && is_post_type_archive( array( $menu_item->object ) ) ) ||
			( 'post_type_archive' == $menu_item->type && is_singular( array( $menu_item->object ) ) )
		) {
			$allow_highlight = true;
			
			// This filter can be used to add some additional logic if needed. E.g. not highlight in some special cases
		    $allow_highlight = apply_filters( 'my_allow_menu_highlight', $allow_highlight, $menu_item, $queried_object, $args );

		    if ( $allow_highlight ) {
			    $menu_item->classes[]                   = 'current-menu-item';
			    $menu_items[ $key ]->current = true;

			    $_anc_id = (int) $menu_item->db_id;

			    while (
				    ( $_anc_id = get_post_meta( $_anc_id, '_menu_item_menu_item_parent', true ) ) &&
				    ! in_array( $_anc_id, $active_ancestor_item_ids )
			    ) {
				    $active_ancestor_item_ids[] = $_anc_id;
			    }

			    $active_parent_item_ids[]   = (int) $menu_item->menu_item_parent;
			    $active_parent_object_ids[] = (int) $menu_item->post_parent;
			    $active_object              = $menu_item->object;
		    }
		}
	}

	// Make sure the IDs are not present multiple times
	$active_ancestor_item_ids = array_filter( array_unique( $active_ancestor_item_ids ) );
	$active_parent_item_ids = array_filter( array_unique( $active_parent_item_ids ) );
	$active_parent_object_ids = array_filter( array_unique( $active_parent_object_ids ) );


	// set parent's class
	foreach ( (array) $menu_items as $key => $parent_item ) {
		$classes = (array) $parent_item->classes;
		$menu_items[$key]->current_item_ancestor = false;
		$menu_items[$key]->current_item_parent = false;

		if (
			isset( $parent_item->type ) &&
			(
				// ancestral post object
				(
					'post_type' == $parent_item->type &&
					! empty( $queried_object->post_type ) &&
					is_post_type_hierarchical( $queried_object->post_type ) &&
					in_array( $parent_item->object_id, $queried_object->ancestors ) &&
					$parent_item->object != $queried_object->ID
				) ||

				// ancestral term
				(
					'taxonomy' == $parent_item->type &&
					isset( $possible_taxonomy_ancestors[ $parent_item->object ] ) &&
					in_array( $parent_item->object_id, $possible_taxonomy_ancestors[ $parent_item->object ] ) &&
					(
						! isset( $queried_object->term_id ) ||
						$parent_item->object_id != $queried_object->term_id
					)
				)
			)
		) {
			$classes[] = empty( $queried_object->taxonomy ) ? 'current-' . $queried_object->post_type . '-ancestor' : 'current-' . $queried_object->taxonomy . '-ancestor';
		}

		if ( in_array(  intval( $parent_item->db_id ), $active_ancestor_item_ids ) ) {
			$classes[] = 'current-menu-ancestor';
			$menu_items[$key]->current_item_ancestor = true;
		}
		if ( in_array( $parent_item->db_id, $active_parent_item_ids ) ) {
			$classes[] = 'current-menu-parent';
			$menu_items[$key]->current_item_parent = true;
		}
		if ( in_array( $parent_item->object_id, $active_parent_object_ids ) )
			$classes[] = 'current-' . $active_object . '-parent';

		if ( 'post_type' == $parent_item->type && 'page' == $parent_item->object ) {
			// Back compat classes for pages to match wp_page_menu()
			if ( in_array('current-menu-parent', $classes) )
				$classes[] = 'current_page_parent';
			if ( in_array('current-menu-ancestor', $classes) )
				$classes[] = 'current_page_ancestor';
		}

		$menu_items[$key]->classes = array_unique( $classes );
	}


	return $menu_items;
}
add_filter( 'wp_nav_menu_objects', 'my_highlight_cpt_archive_and_ancestors_in_menu', 10, 2 );