title: Symfony2 OnetoOne relations
body: <p>I have two class, User and PersonalData. We need relation OnetoOne with Doctrine in Symfony2. In my code I tried this relations, but in MySQL doesnt appear the foreign key. 
  My code:</p>
  
  <pre><code>namespace TFC\UserBundle\Entity;
  
  use Doctrine\ORM\Mapping as ORM;
  
  /**
   * User
   */
  /** @ORM\Entity */
  class User
  {
      /**
       * @Id @Column(type="integer") @GeneratedValue
       */
      private $id;
  
      /**
       * @var string
       */
      private $email;
  }
  
      use Doctrine\ORM\Mapping as ORM;
  
  /**
   * PersonalData
   */
  /** @ORM\Entity */
  class PersonalData
  {
  
       /** @Id @OneToOne(targetEntity="User") */
      private $userId;
  
      /**
       * @var string
       */
      private $firstName;
  }
  </code></pre>