Using Different wp_nav_menu theme_location based on page id (or a parent id)

Question

I am sure this has been asked. It's new territory for me so I don't have the full vocabulary of what I'm doing, nor am I sure this is the right way to do it.

I've got a bunch of internal pages on a site that the only real change from page to page is the tertiary nav I've got in a sidebar div. (And obviously the main content area).

I've got a bunch of theme_locations setup for wp_nav_menu. What's the best way to display them based on page ID or parent ID or something like that?

Something along these lines I found, but it's not quite what I need:

<?php
    $thispageid = get_the_ID();
    if( $thispageid == "1" ) )
        wp_nav_menu( array( 'theme_location' => 'tertiary_surgeons_consulting' ) );
    if( $thispageid == "2" ) )
        wp_nav_menu( array( 'theme_location' => 'tertiary_surgeons_consulting' ) ); 
    else
        wp_nav_menu( array( 'theme_location' => 'tertiary_surgeons_education' ) );
?>

Is this thought process the right way to accomplish this, or am I missing some other way that's already in WordPress?

Answer 1

The best solution would be to allow a user to choose the nav menu location when adding/editing a page. Use a meta box with a dropdown (select) with all the available locations, and then save the selected location in a custom field:

/**
 * Register the meta box.
 */
function wpse_183868_page_nav_menu_meta_box_register() {
    add_meta_box(
        'wpse_183868_page_nav_menu',
        'Nav Menu',
        'wpse_183869_page_nav_menu_meta_box',
        'page',
        'side'
    );
}

add_action( 'add_meta_boxes', 'wpse_183868_page_nav_menu_meta_box_register' );

/**
 * Display the meta box.
 */
function wpse_183869_page_nav_menu_meta_box( $post ) {
    $selected = get_post_meta( $post->ID, '_page_nav_menu', true );
    wp_nonce_field( 'save', 'page_nav_menu_nonce' );
    ?>

<select name="page_nav_menu">
    <?php foreach ( get_registered_nav_menus() as $name => $label ) : ?>
         <option value="<?php echo $name ?>"<?php selected( $name, $selected ) ?>><?php echo $label ?></option>
    <?php endforeach ?>
</select>

<?php
}

/**
 * Save the meta box data.
 */
function wpse_183869_page_nav_menu_save( $post_id ) {
    if ( ! isset( $_POST['page_nav_menu'], $_POST['page_nav_menu_nonce'] ) || ! wp_verify_nonce( $_POST['page_nav_menu_nonce'], 'save' ) )
        return;

    $nav_menus = get_registered_nav_menus();
    $nav_menu = $_POST['page_nav_menu'];

    if ( isset( $nav_menus[ $nav_menu ] ) ) // Paranoid - check the submitted value is definitely valid
        update_post_meta( $post_id, '_page_nav_menu', $nav_menu );
}

add_action( 'save_post', 'wpse_183869_page_nav_menu_save' );

Now in your template you can just use:

if ( ! $page_nav_menu = get_post_meta( get_the_ID(), '_page_nav_menu', true ) )
    $page_nav_menu = 'default_location';

wp_nav_menu( array( 'theme_location' => $page_nav_menu ) );