Submenu Output Using Custom Menus

Using the custom menu builder in wordpress to output submenus with the following code. Useful when you want to add dynamic submenus to the sidebar based on the parent child layout from the menu builder. I prefer this to the old method of assigning parent child relationships on the page settings, since we already have a nice parent child layout using the custom menus.

// add hook
add_filter( 'wp_nav_menu_objects', 'my_wp_nav_menu_objects_sub_menu', 10, 2 );
// filter_hook function to react on sub_menu flag
function my_wp_nav_menu_objects_sub_menu( $sorted_menu_items, $args ) {
  if ( isset( $args->sub_menu ) ) {
    $root_id = 0;
    
    // find the current menu item
    foreach ( $sorted_menu_items as $menu_item ) {
      if ( $menu_item->current ) {
        // set the root id based on whether the current menu item has a parent or not
        $root_id = ( $menu_item->menu_item_parent ) ? $menu_item->menu_item_parent : $menu_item->ID;
        break;
      }
    }
    
    // find the top level parent
    if ( ! isset( $args->direct_parent ) ) {
      $prev_root_id = $root_id;
      while ( $prev_root_id != 0 ) {
        foreach ( $sorted_menu_items as $menu_item ) {
          if ( $menu_item->ID == $prev_root_id ) {
            $prev_root_id = $menu_item->menu_item_parent;
            // don't set the root_id to 0 if we've reached the top of the menu
            if ( $prev_root_id != 0 ) $root_id = $menu_item->menu_item_parent;
            break;
          } 
        }
      }
    }
    $menu_item_parents = array();
    foreach ( $sorted_menu_items as $key => $item ) {
      // init menu_item_parents
      if ( $item->ID == $root_id ) $menu_item_parents[] = $item->ID;
      if ( in_array( $item->menu_item_parent, $menu_item_parents ) ) {
        // part of sub-tree: keep!
        $menu_item_parents[] = $item->ID;
      } else if ( ! ( isset( $args->show_parent ) && in_array( $item->ID, $menu_item_parents ) ) ) {
        // not part of sub-tree: away with it!
        unset( $sorted_menu_items[$key] );
      }
    }
    
    return $sorted_menu_items;
  } else {
    return $sorted_menu_items;
  }
}

Output the submenu, changing the menu parameter value to the menu you wish to use.

wp_nav_menu( array(
  'menu'     => 'Menu Name',
  'sub_menu' => true
) );

If you want the menu to drill down dynamically by filtering the menu based on the direct parent, pass a “direct_parent” => true parameter to the wp_nav_menu call.

wp_nav_menu( array(
  'menu'          => 'Menu Name',
  'sub_menu'      => true,
  'direct_parent' => true
) );

Include the parent page link at the top of the list.

wp_nav_menu( array(
  'menu'        => 'Menu Name',
  'sub_menu'    => true,
  'show_parent' => true
) );

Since this is a filter on the wp_nav_menu, you can use the default parameters from the function as well.

reference: https://christianvarga.com/how-to-get-submenu-items-from-a-wordpress-menu-based-on-parent-or-sibling/

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