base_controller.rb

class Admin::BaseController < ApplicationController
  layout "admin"
  before_filter :require_admin
  
  ActiveScaffold.set_defaults do |config| 
    config.ignore_columns.add [:created_at, :updated_at]
  end
end

categories_controller.rb

class Admin::CategoriesController < Admin::BaseController
  active_scaffold :categories
end

category.rb

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

product.rb

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

products_controller.rb

class Admin::ProductsController < Admin::BaseController
  active_scaffold :products
end

routes.rb

ActionController::Routing::Routes.draw do |map|
    map.namespace :admin do |admin|
    admin.resources :products, :active_scaffold => true
    admin.resources :categories, :active_scaffold => true    
  end
end