products_controller.rb

class ProductsController < ApplicationController
  def preview
    begin
      ActiveRecord::Base.transaction do      
        if request.referer =~ %r{/admin/products/(.+)/edit}
          @product = Product.find($1)
          @product.update_attributes(params[:product])
        else
          @product = Product.new(params[:product])          
          raise "Invalid" unless @product.valid?
          @product.save
        end
        response.body = render :layout => 'application'
        response.status = 200
        @performed_render = true # rails internals
        
        raise 'Changes not saved!'
      end
    rescue => exception
      unless @performed_render
        # Product doesn't exist => Invalid product id
        # Product is valid => Something else should've gone wrong
        if @product.nil? or @product.valid?
          raise exception
        else
          render :text => "Invalid deal:\n #{@product.errors.full_messages.join("\n")}"
        end
      end
    end
  end
end