Conditional sidebar menu

Question

I've created a menu for each section of the site, and I'd like the sidebar to only show the relevant menu to pages in a section, so the 'About' page and its children receive the about menu and the 'Advice' page and its children the advice menu, and so on and so forth.

I've got this working for parent pages but I can't get it working with their children. I've tried various suggestions from around the internet but nothing seems to work correctly.

This code:

<?php if(is_page('advice')){
        wp_nav_menu( array( 'theme_location' => 'advice-menu' ) );
} ?>

gets the correct menu in the correct section, but I need to extend it to cover child pages as well.

All menus are properly registered in both functions.php and Appearance > Menus and both the parent and child pages are all using the same page template.

Answer 1

Bainternet's solution will definitely work, but it you want to use the page slug instead of ID you could put this in your functions.php file:

// GET PAGE ID FROM THE SLUG, HELPER FUNCTION FOR IS_TREE
function get_ID_by_slug($page_slug) {
    $page = get_page_by_path($page_slug);
    if ($page) {
        return $page->ID;
    } else {
        return null;
    }
}

// DETERMINE IF A PAGE IS IN A DESCENDANT TREE
function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
    global $post;         // load details about this page
    $pid = get_ID_by_slug($pid);
    if( is_page() && ($post->post_parent==$pid || is_page($pid) ) )
               return true;   // we're at the page or at a sub page
    else
               return false;  // we're elsewhere
};

And then use this for the conditional statement in your sidebar.php or whatever:

<?php if ( is_tree('advice') ) { 
     wp_nav_menu( array( 'theme_location' => 'advice-menu' ) );
} elseif ( is_tree('another-page-slug') ) { 
     wp_nav_menu( array( 'theme_location' => 'another-page-menu' ) );
} ?>

That will return any page that is within the 'tree' - both the parent page itself and any children below it. Hope this helps, best of luck!

Answer 2

I have a function i made a while a go that will help you:

/**
 * Function to check if the current page/ custom is a child of a given one
 *
 * @param (int) - parent id to check
 *
 * @return (bool) - if it is a child true else it returns false
 */
function is_child_of($parent_id) {
    global $post;
    $ps = get_post_ancestors( $post );
    if (empty($ps)) return false;
    return (in_array($parent_id,$ps));
}

The only down side is that you must know the parent page's ID, if not the I can extend it a bit to work with page slug as well.

Anyway once you have this function in place you can change your conditional check to something like this:

<?php if(is_page('advice') || is_child_of(12)){
        wp_nav_menu( array( 'theme_location' => 'advice-menu' ) );
} ?>

assuming that the id of advice is 12 :)