Force ancestor page for another page in wordpress - php

Question

My customer has a Wordpress blog with a page named "Galleries" that serves as a kind of main page as he manually writes there links to other pages, both local and external.
The main menu for the blog has also a link to this Galleries page. Problem is that the menu item only is active for that page, but not for the linked local pages, unless you previously selected Galleries as their parent page.
So I was wondering if it will be possible (with a hook) to virtually force an ancestor to a page that meets certain conditions before the menu is rendered, avoiding the need to manually edit every page and make it a child to "Galleries"

Answer 1

You'd have to find a filter that works in the very early stage of assembling the menu data, or do some heavy array juggling once it's already build.
I'd suggest another approach:
Update the parent for all currently existing pages via a simple database query. wp_posts contains a column post_parent - so create a query that filters out all of your pages that satisfy the criteria you mentioned (how easy or hard that will be, depends on what your criteria are - might involve JOINs with other tables like wp_postmeta, wp_terms), and then update the parent for those. This is a one-time thing, so it can be done via phpMyAdmin or some database frontend like that. Keep in mind that since "pages" are ordinary posts internally, and stored in the same table, you will have to filter for only those posts that are pages in the first place.
Create a function that hooks into the post save/update actions, in there again check if the criteria are matched, and if so set the parent page for the post at that point in the PHP data structure, before it gets written away to the database.

Answer 2

Finally found how to do it:
$main_gallery_page= get_page_by_title('Galleries');
add_filter('nav_menu_css_class' , 'force_ancestor_nav_class' , 10 , 2);
function force_ancestor_nav_class($classes, $item){
global $main_gallery_page,$post;
//condition: could be any condition: by category, title, contents, ...
$make_ancestor=substr( $post->post_title, 0, 7 ) === "Gallery";
//only adds the class if the condition is met and not already present
if ($item->object_id==$main_gallery_page->ID && $make_ancestor && !in_array('current-page-ancestor',$classes)) {$classes[]='current-page-ancestor';}
return $classes;
}