title: Using Rails 3.1 :as => :admin for updating attributes protected by attr_accessible
body: <p>After reading about <a href="http://api.rubyonrails.org/classes/ActiveModel/MassAssignmentSecurity/ClassMethods.html#method-i-attr_accessible" rel="noreferrer">attr_accessible in the Rails 3.1 API</a>, I see that there is an as <code>:admin</code> option in there. I would like to know two things.</p>
  
  <ol>
  <li><p>If the user has an admin flag, how do does my controller tell my model that the user is an admin.</p></li>
  <li><p>If the user is an owner, can i specify <code>:as =&gt; owner</code> in my model, and once again how does my controller inform my model they are the owner of an item.</p></li>
  </ol>