title: Doctrine - Missing value for primary key
body: <p>I have this entity:</p>
  
  <pre><code>/**
   * Class ParserURLs
   * @package AppBundle\Entity
   * @ORM\Entity(repositoryClass="AppBundle\Entity\ParserURLsRepository")
   */
  class ParserURLs
  {
  
  /**
   * @ORM\Id
   * @ORM\Column(type="string", length=50)
   */
  protected $external_id;
  
  /**
   * @ORM\Id
   * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Parser")
   */
  protected $parser;
  
  ...
  }
  </code></pre>
  
  <p>Any I try to get results filtering by $parser, like this:</p>
  
  <pre><code>$query = $this-&gt;doctrine-&gt;getManager()-&gt;createQuery(
              'SELECT u
              FROM AppBundle:ParserURLs u
              WHERE u.parser = :parser')
  -&gt;setParameter('parser', $this-&gt;parser);
  
  $result = $query-&gt;getResult();
  </code></pre>
  
  <p>But Doctrine tell me: <code>Missing value for primary key parser on AppBundle\Entity\ParserURLs</code></p>
  
  <p>I think that the problem es because the multiple primary key <strong>external_id-parser</strong> but now, I only want search by one of their. It's not possible in Doctrine? I need this primary keys.
  Thanks you in advance</p>