Display child page content instead of parent

Hello,

this post is not a question, but rather an explanation of how I managed to find a solution to my problem. Perhaps this will be useful for someone in the future.

My problem was quite a common one: I have a site structure which looks like this:
> Page A
> Page B
>> Subpage B.1
>> Subpage B.2
> Page C

What I needed to achieve was to load the contents of Subpage B.1 instead of Page B when an user clicked on “Page B” link in the menu.

There are a few solutions which use WP_Redirect, but I find them a bit messy + I can’t use a special template because my site has to be manageable by people who don’t know what “template” means (: Other way would be to create a check and a new query in my main page template file, but any way I looked at it, it always seemed like I needed to write my design stuff twice — once for non-subpages and once for subpages.

So I investigated modifying the original WP_Query and long story short, this is how I’m doing it (it all goes to functions.php):

// get first child id. returns false if no children exist. useful in other places as well.
function first_child($pid) {
	$children = get_pages("child_of=".$pid."&sort_column=menu_order");
	if ($children) {
		$firstchild = $children[0];
		return $firstchild->ID;
	} else {
		return false;
	}
}

// modify the page main query
add_action( 'pre_get_posts', 'replace_content_with_child' );
function replace_content_with_child( $query ) {
	$firstchild = first_child($query->queried_object_id);

	if( $query->is_main_query() && !is_admin() && is_page() && $firstchild ) {
		$query->query_vars['page_id'] = $firstchild;
	}

}

I’m not sure how “cheap” it is (should be quite?), but it keeps the template pages clean and seems to work well so far.

I’m not a very professional developer, so if anyone happens to see errors or points of improvement (speed wise?!), all suggestions are welcome (: