title: @ORM\Id does not make an identifier for an entity in symfony
body: <p>I hava a class City which consists of a number of traits.</p>
  
  <pre><code>/**
   * @ORM\Entity
   * @ORM\Table(name="City")
   * @SoftDeleteable(fieldName="deletedAt")
   */
  class City
  {
      use IdentifiableEntity;
      use TimestampableEntity;    
  }
  </code></pre>
  
  <p>I have a trait IdentifiableEntity</p>
  
  <pre><code>trait IdentifiableEntity
  {
      /**
       * @var integer
       * @ORM\Column(name="id", type="integer", options={"unsigned":true})
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      protected $id;
      //setter getter
  }
  </code></pre>
  
  <p>the problem occurs when I execute <strong>bin/console doctrine:migrations:diff</strong> I get this error message </p>
  
  <pre><code>[Doctrine\ORM\Mapping\MappingException]                                                                                         
    No identifier/primary key specified for Entity "AppBundle\Entity\City\City". Every Entity must have an identifier/primary key. 
  </code></pre>
  
  <p>Why is that? I have clearly stated in my trait that <code>$id</code> should be treated like <code>@ORM\Id</code>. Adding <code>@ORM\GeneratedValue(strategy="IDENTITY")</code> does not help either. What's wrong?</p>
  
  <p>P.S. I am using symfony 3.2 and doctrine/doctrine-migrations-bundle: 1.0</p>