Devise + Rails 3.0.4 ends session after AJAX request

I have an action triggered by an AJAX request generated by Ajax.InPlaceEditor or InPlaceCollectionEditor like this:

new Ajax.InPlaceCollectionEditor('agent_email', 'inspections/<%= @inspection.id %>/update_field', 
{
collection: [<% @agents.each do |agent| %>
        '<%= agent.email %>',           
        <% end %>],
    okText: 'Update',
    cancelText: 'Never mind',
    savingText: 'Updating...'

});

At the other end, the action contains this:

def update_field
  --some code here--
  if success
    puts "stored change"
    render :text => result
  else
    puts "did note change store"
    render :text => inspection.errors.to_json, :status => 500
  end
end

Once any of the render methods are reached, the session expires, and next time the user send a request, Devise sends them to the logon on page.

Even though I am exempting update_field from authentication (before_filter :authenticate_user!, :except => :update_field), the session is still getting reset.

I have looked at the answer at a very similar question at Devise session immediately expiring on .js call [AJAX], but it is not solving my particular problem.

Any ideas?

I got this to work by getting the code from http://weblog.rubyonrails.org/2011/2/8/csrf-protection-bypass-in-ruby-on-rails (prototype-snippet.js):

/*
 * Registers a callback which copies the csrf token into the
 * X-CSRF-Token header with each ajax request.  Necessary to 
 * work with rails applications which have fixed
 * CVE-2011-0447
*/

Ajax.Responders.register({
onCreate: function(request) {
  var csrf_meta_tag = $$('meta[name=csrf-token]')[0];

  if (csrf_meta_tag) {
    var header = 'X-CSRF-Token',
        token = csrf_meta_tag.readAttribute('content');

    if (!request.options.requestHeaders) {
      request.options.requestHeaders = {};
    }
    request.options.requestHeaders[header] = token;
  }
}
});

... within a Javascript block in my application.html.erb:

<script type="text/javascript">
  (... the code from above)
</script>

Also don't forget to add:

<%= csrf_meta_tag %>

in the same file towards the top (if not already there).

The document "CSRF Protection Bypass in Ruby on Rails" explains why this works.