title: Fields getting removed when updatating mongodb document
body: <p>I'm new to mongoDB and I have stumbled over some update issues when using <code>Save()</code>.</p>
  
  <p>When I click the save-button in my html-page I run the following code:</p>
  
  <pre><code>&lt;?php
  $action = (!empty($_POST['btn_submit']) &amp;&amp;
          ($_POST['btn_submit'] === 'Save')) ? 'save_article' : 'show_form';
  $id = $_REQUEST['id'];
  try {
      $mongodb = new Mongo();
      $articleCollection = $mongodb-&gt;myblogsite-&gt;articles;
  } catch (MongoConnectionException $e) {
      die('Failed to connect to MongoDB ' . $e-&gt;getMessage());
  }
  switch ($action) {
      case 'save_article':
        $article = array();
          if (!empty($id)) {
              $article['_id'] = new MongoId($id);
          } else {
              $article['_id'] = new MongoId();
          }
          $article['title'] = $_POST['title'];
          $article['content'] = $_POST['content'];
          $article['tags'] = $_POST['tags'];
  
          if (!empty($id)) {
              $article['updated_at'] = new MongoDate();
          } else {
              $article['saved_at'] = new MongoDate();
          }
  
          $articleCollection-&gt;save($article);
          break;
      case 'show_form':
      default:
          if (!empty($id)) {
              $article = $articleCollection-&gt;findOne(array('_id' =&gt; new MongoId($id)));
          }
  }
  ?&gt;
  </code></pre>
  
  <p>The idea is that if <code>$id</code> is not empty I update the document or else I insert.
  I use <code>save()</code> to mange that.</p>
  
  <p>I want two date-fields. One that hold the date the document was made and a new one for when it was last updated. 
  The <code>“save_at”</code> can not be updated. The <code>“updated_at”</code> change every time an update is made. 
  My problem now is that when I try to add the <code>“updated_at”</code> the <code>“save_at”</code> gets removed.
  I also update the <code>“saved_at”</code>-field. </p>
  
  <p><strong>My questions is:</strong></p>
  
  <p>How do I insert an new field <code>('updated_at')</code> without removing the <code>“saved_at”</code>-field?</p>
  
  <p>How do I keep the date in <code>“saved_at”</code>-field as it is without updating its value?</p>