title: symfony2 setter for related entity id
body: <p>My relation in entity:</p>
  
  <pre><code> /**
   * @ORM\ManyToOne(targetEntity="Group")
   */
  protected $group;
  </code></pre>
  
  <p>now I have a setter method <code>setGroup()</code> to set related entity, but there seems no method <code>setGroupId()</code> to set <code>group_id</code> without the entity object. How can I set <code>group_id</code> directly?</p>