title: doctrine 2 query builder and join tables
body: <p>I'm trying to get all comments for each post in my home page </p>
  
  <pre><code>return 
  $this-&gt;createQueryBuilder('c')
  -&gt;select('c')
  -&gt;from('Sdz\BlogBundle\Entity\Commentaire' ,'c')                
  -&gt;leftJoin('a.comments' ,'c')-&gt;getQuery()-&gt;getResult() ;
  </code></pre>
  
  <p>but I'm getting this error</p>
  
  <pre><code>[Semantical Error] line 0, col 58 near '.comments c,': Error:
  Identification Variable a used in join path expression but was not defined before.
  </code></pre>
  
  <p>PS : The mapping is correct because I can see the page article with its comments.</p>