title: WP Query: Exclude ALL terms of a custom taxonomy
body: <p>In WordPress I have a custom post type 'books' and two custom taxonomies 'genre' and 'series'. While all books have a genre, not all are part of a series. I want now to be able to query all non-series titles, i.e. all books w/o the series taxonomy. I hit the WordPress forums next and also googled for a solution, but only found how to exclude specific terms of a custom taxonomy but not the custom taxonomy itself and all terms that belong to it. </p>
  
  <p>Of course, I could simply list all terms from 'series' in my tax query to exclude them, but if I add new terms for 'series' in the future, I have to remember to edit my query and I like to avoid it. That's why I came up with the following idea but it's not working:</p>
  
  <pre><code>&lt;?php
  $terms = get_terms( 'series', $args );
  $count = count( $terms );
  $i = 0;
  foreach ( $terms as $term ) {
      $i++;
      $term_list .= "'" . $term-&gt;slug . "'";
      if ( $count != $i ) {
          $term_list .= ', ';
      }
      else {
          $term_list .= '';
      }
  }
  $args = array(
      'post_type' =&gt; 'books',
      'order' =&gt; 'ASC',
      'orderby' =&gt; 'date',
      'posts_per_page' =&gt; '-1',
      'tax_query'        =&gt; array(
      array(
          'taxonomy'  =&gt; 'series',
          'terms' =&gt; array($term_list),
          'field' =&gt; 'slug',
          'operator'  =&gt; 'NOT IN')
          ),
  );
  query_posts($args);?&gt;
  </code></pre>
  
  <p>As you can see I tried to query first all terms for 'series' and output them into a list in the style it would have to go into the tax array. The result I currently get is that all books appear when the query runs.</p>
  
  <p>Can someone tell me where I went wrong? Or if you have another approach to exclude all terms of a custom taxonomy w/o manually tweaking the code every time a new term is added, I'm all ears.</p>