title: Symfony2 : missing an assigned ID
body: <p>I have a Place and a Placetype entity.
  My Place entity have his id attribute set to AUTO:</p>
  
  <pre><code>/**
   * @var integer $id
   *
   * @ORM\Column(name="id", type="integer", nullable=false)
   * @ORM\Id
   * @ORM\GeneratedValue(strategy="AUTO")
   */
  private $id;
  </code></pre>
  
  <p>But when I want to persist an object, I got the following error : </p>
  
  <blockquote>
    <p>Entity of type blabla\Entity\Place is missing an assigned ID. The
    identifier generation strategy for this entity requires the ID field
    to be populated before EntityManager#persist() is called. If you want
    automatically generated identifiers instead you need to adjust the
    metadata mapping accordingly.</p>
  </blockquote>
  
  <p>Do you know how to fix this?</p>
  
  <p>Thanks for your help</p>