title: Doctrine2: Call to undefined method Doctrine\ORM\QueryBuilder::field()
body: <p>I am trying to create a query with the doctrine query builder:</p>
  
  <pre><code>public function searchAuthors($q)
  {
      $qb = $this-&gt;createQueryBuilder('Author')
      -&gt;field('person.firstname')-&gt;equals('test');
  
      return $qb-&gt;getQuery()-&gt;getResult();
  }
  </code></pre>
  
  <p>But I am getting : </p>
  
  <pre><code>Fatal error: Call to undefined method Doctrine\ORM\QueryBuilder::field() in ... on line 18 
  </code></pre>
  
  <p>The field() method is from the doctrine documentation webpage. Could you please help me to resolve that.</p>
  
  <p>Thank you.</p>