title: Symfony - form event - get new and old data
body: <p>I have a <code>Post</code> entity and <code>PostType</code>.</p>
  
  <p>Post entity have field <code>$requireModeration</code> it means someone must check this post and approve it before it can be published.</p>
  
  <p>When I'm editing post I want to compare old post value and new post value. And if value is changed I also want to change <code>requireModeration</code> flag to true.</p>
  
  <p>Can I do it through form events?<br>
  Something like this:</p>
  
  <pre><code>public function postSubmit(FormEvent $event)
  {
      $newPost = $event-&gt;getData();
      $newContent = $post-&gt;getContent(); // return new contant of post
      $oldPost = ... // here I want to get old post
      $oldContent = $oldPost-&gt;getContent();
  
      if($newContent != $oldContent) {
          // ...
      }
  }
  </code></pre>
  
  <p>But unfortunately I can get only new, just sent data through FormEvent object. </p>