title: Updating "expires" with mongoose
body: <p>I am new to mongodb and mongoose, and I am having a hard time figuring out how to do the following.
  I have a Schema, say "User", which includes a field like this:</p>
  
  <pre><code>createdAt  : {type: Date, required: true, default: Date.now, expires: '4h'}
  </code></pre>
  
  <p>Therefore, when I create a new user, say <code>var user=new User()</code> (assume User <code>requires</code> the correct model), the new user created will be deleted after (approximately) 4h (if I understood it correctly).</p>
  
  <p>I was wondering if there is a way to update the <code>expires</code> property of a user. I would like to do the following:</p>
  
  <ul>
  <li>Retrieve the user from my database</li>
  <li>Update <code>expires</code>, such that the user does NOT expire anymore</li>
  <li>Save the changes</li>
  </ul>
  
  <p>Any suggestions will be highly appreciated!</p>