title: SQL query in symfony2 and doctrine2
body: <p>i'm trying to create a SQL query in Doctrine 2 (Symfony2 Framework), and i need some help.
  I have two Tables Movie and Person in relation many to many in table Movie_Person.</p>
  
  <p>I want to get a Persons who plays in film. So clear SQL should look like:</p>
  
  <pre><code>SELECT p.* FROM Person p WHERE EXISTS 
                  (SELECT m.* FROM Movie_Person m WHERE p.id = m.person_id)
  </code></pre>
  
  <p>How to write this in Symfony2 + Doctrine2 ?</p>