title: Left Join with NOT IN clause and QueryBuilder
body: <p>i'm trying to convert a query to DQL or build ir with query builder.
  I can retrive with success the results from database with the following query:</p>
  
  <pre><code>SELECT * FROM flag 
  left join countries
  on flag.id = countries.flag
  where countries.flag IS NULL
  </code></pre>
  
  <p>Now, i need to conver this to DQL or build it with queryBuilder.
  My entities are Country and Flag. The entity Country as a field flag as a external id.</p>
  
  <p>So far, i got the following code:</p>
  
  <pre><code> $qb = $em-&gt;createQueryBuilder();
      $available =
          $qb
          -&gt;select('f')
          -&gt;from('Flag', 'f')
          -&gt;leftJoin('Countries', 'c', 'ON', 'c.flag = f.id')
          -&gt;where('f.id IS NULL');
  </code></pre>
  
  <p>My dump of $available doesnt return anything.
  What's whong with this QueryBuilder?</p>
  
  <p>Thanks.</p>