WordPress nav menu custom current URL ancestor

functions.php

<?php

/**
 * @see https://github.com/WordPress/WordPress/blob/3.9.1/wp-includes/nav-menu-template.php#L351-L358
 */
function prefix_wp_nav_menu_objects( $sorted_menu_items, $args ) {
	// @see http://wordpress.stackexchange.com/a/126534
	$current_url = home_url( add_query_arg( null, null ) );

	foreach ( $sorted_menu_items as $menu_item ) {
		if ( strpos( $current_url, $menu_item->url ) === 0 ) {
			$menu_item->classes[] = 'prefix-current-url-ancestor';
		}
	}

	return $sorted_menu_items;
}

add_filter( 'wp_nav_menu_objects', 'prefix_wp_nav_menu_objects', 10, 2 );