Shopping cart : handling no product error with rails / ajax

I am using ajax to change the displayed cart as the user add product to cart. To do so i call with :remote=>true the correct controller that send back the updated cart. The controller method to do so is the following :

def create
@cart = current_cart
product = Product.find(params[:product_id])
@line_item = @cart.add_product(product.id)
respond_to do |format|
  if @line_item.sa开发者_运维问答ve
    format.html { redirect_to(magasin_url) }
    format.js   {@current_item = @line_item }
    format.xml  { render :xml => @line_item, :status => :created, :location => @line_item }
  else
    format.html { render :action => "new" }
    format.xml  { render :xml => @line_item.errors, :status => :unprocessable_entity }
  end
end

end

I'd like to be able to handle an empty stock by sending a notice message instead of updating the cart.

I tried this :

if product.stock_to_display <=0
  respond_to do |format|
    format.html { }
    format.js   {}
  end
end

But i do not know what to put in the format. and i have no idea how to do a condition in the rjs :

    page.replace_html('panier', render(@cart))

page[:panier].visual_effect :blind_down if @cart.total_items == 1

page[:current_item].visual_effect   :highlight,
                                    :startcolor => "#88ff88",
                                    :endcolor => "#114411"

page.select("#notice").each {|notice| notice.hide}

So i'd like to know how to handle the error with ajax/rjs. Thanks

---

Ok after a little bit of try i managed to do something that seems clean.

I created a @success boolean in the controller and affected him a value, after that i can use the boolean in RJS to either do my stuff or display the notice. Here is my new RJS.

if @success
  page.replace_html('panier', render(@cart))
  page[:panier].visual_effect :blind_down if @cart.total_items == 1
  page[:current_item].visual_effect     :highlight,
                                        :startcolor => "#88ff88",
                                        :endcolor => "#114411"
  page.select("#notice").each {|notice| notice.hide}
else
  page.replace_html :notice, flash[:notice]
  flash.discard
end

If you have a javascript error with the display, check the application layout as it may be restricted by a "if notice" condition