title: Symfony2 and Doctrine: complex relations
body: <p>In my project I have entities <code>News</code>, <code>Articles</code> and <code>Comments</code>.
  Users can add comments both to articles and news. Each user can add a few comments for each article or new.
  I want have single <code>Comments</code> entity with composite key for relationships with <code>News</code> and <code>Articles</code> like</p>
  
  <p><code>subj_type</code> (news or articles) + <code>subj_id</code> (id of new or article)</p>
  
  <p>Can I organize relationships by this way? Or I must organize two different Comments entities (like <code>NewsComments</code> and <code>ArticlesComments</code>)?</p>