CSS class on last post in loop ( custom query )

Question

I have been using this question to try to do this but I am having some difficutlty. I have a custom post type, media, and I am working on the archive-media.php template. I have it partially working in that when I delete the if condition out of the code it does add the class, but of course to every post. I have determined that $wp_query->current_post is not returning a value, so I feel I must not be using this right. I have the code in my functions.php file. The codex says that current_post is available in the loop, so I am a bit confused.

add_filter('post_class', 'my_post_class');
function my_post_class($classes){
  global $wp_query;
  if(($wp_query->current_post+1) == $wp_query->post_count) $classes[] = 'last';
  return $classes;
}

Answer 1

I realised that $wp_query doesn't work for where I had customised the queries with WP_query. This is completely obvious I suppose when you know, but I didn't, so here is the adjusted code in case it benefits some other amateur! Note that $media_query, $events_query etc are the instances of WP_query which I had created myself on the respective archive pages when I did the custom queries, so you'd need to do your own versions of these using whatever you used for the queries. I say this in case someone thinks these were defined by wordpress and try to copy and paste. ( $wp_query itself on the other hand of course is. )

// Add css class 'last' to last posts in archive query [ used in conjunction with post_class('archive-post') ]
add_filter('post_class', 'last_post_class');
function last_post_class($classes) {
    // Regular Archives
  global $wp_query;
  if(($wp_query->current_post+1) == $wp_query->post_count)
        $classes[] = 'last';

    // Media Archive
    global $media_query;
  if(($media_query->current_post+1) == $media_query->post_count)
        $classes[] = 'last';

    // Events Archive
    global $events_query;
  if(($events_query->current_post+1) == $events_query->post_count)
        $classes[] = 'last';

    // Author Archive
    global $author_query;
  if(($author_query->current_post+1) == $author_query->post_count)
        $classes[] = 'last';

  return $classes;
}