title: Rails 3 : Increment Manually a field with mongoid
body: <p>i'm begining with the gem mongoid in rails3 .</p>
  
  <p>I want to know how i can increment manually a field containing an integer.</p>
  
  <p>I've tried a lot of method but no one work well.</p>
  
  <p>And i'd like to know if this action can be done in an atomic context ? </p>