title: Symfony forms - partial object update
body: <p>Given a typical Doctrine entity used to manage say user accounts, suppose I want to provide registered users with a form which consists solely of a field used to modify their professional bio. This means <em>only</em> the description would be passed back to the <code>update</code> action:</p>
  
  <pre><code>public function updateAction(Project $project, Request $request)
  {
    $user = SOME_RETRIEVED_USER_OBJECT;
    $form = $this-&gt;createForm(new UserForm(), $user);
    $form-&gt;bindRequest($request);
    ...
  }
  </code></pre>
  
  <p>However, after executing <code>bindRequest</code>, all of the <code>$user</code> attribute values <em>except</em> for the bio are empty. Surely there is some way to perform a partial object update in Symfony?</p>