Problem adding 'has-children' class to wp_nav_menu

Question

According to a number of references online, I should be able to use the following function to add the class, has-children, to menu items that have children under them:

function gtp_nav_menu_css_class( $css_class, $item ) {
    global $wpdb;
    $has_children = $wpdb->get_var("SELECT COUNT(meta_id) FROM wp_postmeta WHERE meta_key='_menu_item_menu_item_parent' AND meta_value='" . $item->ID . "'");
    if ($has_children > 0) {
        array_push($css_class, 'has-children');
    }
    return $css_class;
}
add_filter( 'nav_menu_css_class', 'gtp_nav_menu_css_class', 10, 4 );

I am using wp_nav_menu() in my header.php. However, after adding this code to functions.php, I am not seeing the has-children class being added to any menu items (when in fact there are menu items that contain children). Does anybody know what may be the culprit?

Notes:

• I am using WP 3.4.1.
• I am running a Multisite setup
• Table prefix is 'wp_'
• Via debug echo statements, I can see that the function is getting executed, but not inside the conditional statement.

Answer 1

You can't shouldn't hardcode table names into queries, especially with multisite. Swap $wpdb->postmeta for wp_postmeta and it'll likely work.

edit- technically you can, edited for clarity.