I've got in-place editing on a page in my app (using Rails 2.3.5 and jQuery). I want to know how to display an error on the page when the update fails.

I'm using ajax (an XMLHttpRequest) to save an update to a Comment object. The controller has an update method like this:

def update
  @comment = Comment.find(params[:id])
  respond_to do |format|
    # if @comment.update_attributes!(params[:comment])
    if false #deliberately forcing a fail here to see what happens
      format.json { render :nothing =>  true }
    else
      format.json { render :json => @comment.errors, :status => :unprocessable_entity }
    end
  end
end

In Firebug, I can see the server returns a "422" (an appropriate validation error status code). But it's a response to an XMLHttpRequest so there is no redirect to an error page.

I think I actually want to do this:

format.json { render :json => @comment.errors}

or maybe this:

format.json {render :json => { :status => :error, :message => "Could not be saved" }.to_json, :status => 400 }

and trigger some Javascript function that iterates through (and displays) any errors.

I'm using a rails plugin http://github.com/janv/rest_in_place/ to implement the in-place editing. It doesn't appear to have any callback function to handle a failure. What are my options? Can I write some Javascript to respond to a failure condition without hacking the plugin? Do I have to hack the rest_in_place plugin to handle a failure condition? Is there a better plugin (for Rails or jQuery) that handles in-place editing, including failure conditions?

UPDATE

This post from Peter Bui (http://paydrotalks.com/posts/45-standard-json-response-for-rails-and-jquery) was helpful in showing how to handle an error message from the server using XMLHttpRequest.status. I looked at his implementation of a blog using ajax (http://github.com/paydro/talks). I'm surprised at the complexity required to handle a simple error condition. Usually Rails has all the goodness baked in but it seems server errors with JSON are out of scope. Can that be?

I also looked at grimen's validatious-on-rails (http://github.com/grimen/validatious-on-rails/) which accommodates models validations when ajax XMLHttpRequest is used. It's not clear to me how I'd use it to handle the general case of a "save" failing when validations succeed.

P.S.

Please vote me up... so I can use more than one HTML link when I ask my question :-)