title: posts_orderby filter with multiple meta values - Wordpress 3.9.1
body: <p>There is the problem...</p>
  
  <p>I have a custom post type with multiple meta_values (like years, collection number, pages, ...)</p>
  
  <p>I want to order the post in this order : year, collection number, pages</p>
  
  <p>There is my code</p>
  
  <pre><code>$args = array(
      'posts_per_page' =&gt; 20,
      'paged' =&gt; $paged,
      'orderby'    =&gt; 'meta_value_num',
      'meta_key'   =&gt; '_year'
  );
  
  add_filter('posts_orderby','orderbyreplace');
  $wp_query = new WP_Query($args);
  remove_filter('posts_orderby','orderbyreplace');
  </code></pre>
  
  <p>In my functions.php</p>
  
  <pre><code>function orderbyreplace($orderby) {  
      $orderby = '_year.meta_value+0 DESC, _collection_number.meta_value+0 DESC, _pages.meta_value+0 DESC';
  
      return $orderby;
  }
  </code></pre>
  
  <p>But when I applied the filter, I have 0 result.</p>
  
  <p>My wordpress version is 3.9.1</p>
  
  <p>My meta values is _year, _collection_number and _pages</p>
  
  <p>Any idea ?</p>