title: mongodb second id field with auto increment
body: <p>I want to have an additional ID field in my mongodb collection.
  The objectId is perfect for get unique IDs, but I need shorter IDs for my user management. These IDs should look like <code>100001</code>, <code>100002</code> and so on. Is it possible to get these by auto increment?</p>
  
  <p>Thx</p>