Rails - Aug 2006 - AJAX question

Dear all,

Help me get a simple AJAX behavior right. I know zero AJAX, thus  
please advice. Below is a simplification of the real code and I hope  
I can describe my intention clearly.

class StoreController < ApplicationController
   def show_cart
     @cart = Cart.new
     @new_item = CartItem.new
   end

   def add_item
     @new_item = CartItem.new(params[:new_item])
     if @new_item.valid?
       @cart.items << @new_item
       # execute add_item.rjs
     else
       breakpoint
       render :partial => ''new_item''
     end
   end
end

app/views/store/show_cart.rhtml

<h1>Cart contents</h1>

<div id="cart">
   <%= render :partial => ''cart'', :object => @cart
%>
</div>

<div id="new-item-form">
   <%= render :partial => ''new_item'', :object =>
@new_item %>
</div>

app/views/store/_new_item.rhtml

<%= form_remote_tag :url => { :action => ''add_item'' }
%>
   <%= error_messages_for ''new_item'' %>
   <%= text_field ''new_item'',
''product_code'' %>
   <%= text_field ''new_item'', ''quantity''
%>
   <%= submit_tag ''Add Item'' %>
<%= end_form_tag %>

Please remember that the code is chopped in various places but I know  
for sure you guys with high Rails-fu skill know the scenario.

The full scenario:
- I want to hit the breakpoint that I put in StoreController. The  
validations in CartItem have been set to not let @new_item valid.
- When I hit the breakpoint in StoreController, I manually tipe  
render :partial => ''new_item'' and Rails is always smart
enough to
return the right partial, complete with the error_messages_for result  
and fieldWithErrors
- If I remove the breakpoint and start testing with a browser, the  
browser IS NOT refreshing the ''new-form-item'' div. Why of
course!?!

Question is: what is the best way to reload the div thus the user can  
see the error messages on their browser? Should I add << :update =>  
''new-item-form'' >> to form_remote_tag argument? This is
what I''m
doubting, is it the right way? Is there a better way?

Thanks in advance.

Sincerely,
John