title: Find all entities without a relation
body: <p>My question heavily depends on this: <a href="https://stackoverflow.com/questions/7030480/doctrine-query-only-where-relationship-doesnt-exist">Doctrine: Query only where relationship doesn&#39;t exist?</a></p>
  
  <p>I wonder how the very same thing can be accomplished using Doctrine2. I am especially interested in good practices. Do I have to use the <code>QueryBuilder</code> or is it possible to use a <code>findBy</code> on a repository?</p>