title: Doctrine subquery in symfony
body: <p>I've got a little issue. I've got to do a query that is based on a subquery.
  The SQL query I need to get to is:</p>
  <pre><code>FROM testing t
  WHERE t.id NOT IN (
      SELECT h.task_id
      FROM task t
      INNER JOIN hardware h ON h.task_id = t.id
            );
  </code></pre>
  <p>What I have for a moment is the basic query and I don't really understand how to do that subquery..</p>
  <pre><code>$test-&gt;createQueryBuilder('t')
              -&gt;select('t.id')
              -&gt;getQuery()-&gt;getArrayResult();
  </code></pre>