How to set menu choice as current in certain pages

OK, try adding this PHP Snippet:

add_filter( 'wp_nav_menu_objects', 'add_menu_parent_class' );
function add_menu_parent_class( $items ) {

    $parents = array();
    foreach ( $items as $item ) {
        if ( in_array('current-page-ancestor', $item->classes)  ) {
            $parents[] = $item->menu_item_parent;
        }
    }

    foreach ( $items as $item ) {
        if ( in_array( $item->ID, $parents ) ) {
            $item->classes[] = 'current-menu-ancestor'; 
        }
    }

    return $items;    
}

If that works it should add the current-menu-ancestor class to the top level item.

Let me know :)

Wow, it works and more sophisticated & handy too, thanks.

Glad to be of help!
For reference i saved the snippet in this GIST:
https://gist.github.com/diggeddy/dcfb6a55c9537eb1c233b7b1069b9af2

BUT it not only checks for pages, it will check for posts in a similar scenario eg. Posts in specific Categories that are listed a sub menu.