title: WooCommerce Custom Product Column Sortable Backed not working
body: <p>I have a custom <code>postmeta</code> field which stores the post/product_id in a serialized way. Example <code>_related_ids</code> => <code>a:4:{i:0;i:2462;i:1;i:2466;i:2;i:2469;i:3;i:2472;}</code></p>
  
  <p>I'm showing the product_id count of <code>_related_ids</code> in WooCommerce Product Listing screen (Backed), which is working fine. Now I want to make that column sortable. So I write a function <code>related_product_col_sort</code> which is hooked into <code>manage_edit-product_sortable_columns</code>. Column Shorting is not working. <em>(not working means it is not ordering the product_id count properly)</em>.</p>
  
  <p>Here is my full code </p>
  
  <pre><code>//_related_ids =&gt; a:4:{i:0;i:2462;i:1;i:2466;i:2;i:2469;i:3;i:2472;}
  //manage_product_posts_custom_column
  add_filter('manage_edit-product_columns', 'related_product_col');
  
  function related_product_col($columns) {
      $new_columns = (is_array($columns)) ? $columns : array();
      $new_columns['RELATED'] = 'Related Product';
      return $new_columns;
  }
  
  add_action('manage_product_posts_custom_column', 'related_product_col_data', 2);
  
  function related_product_col_data($column) {
      global $post;
      $related_product_ids = get_post_meta($post-&gt;ID, '_related_ids', true);
      if ($column == 'RELATED') {
          if (isset($related_product_ids) &amp;&amp; !empty($related_product_ids)) {
              echo count($related_product_ids);
          } else {
              echo "--";
          }
      }
  }
  
  add_filter("manage_edit-product_sortable_columns", 'related_product_col_sort');
  
  function related_product_col_sort($columns) {
      $custom = array(
          'RELATED' =&gt; '_related_ids'
      );
      return wp_parse_args($custom, $columns);
  }
  </code></pre>
  
  <p>Can anyone help me out with the correct logic/code with <code>related_product_col_sort</code> function.</p>
  
  <p>Thanks.</p>