categories_controller.rb

  def create
    user = User.find_by_login(current_user.login)
    @category = Category.new(params[:category])
      if user.categories.create params[:category]
        flash[:notice] = 'Category successfully added !'
        redirect_to(categories_url)
      else
        flash[:notice] = 'There was an error processing your request'
        redirect_to(categories_url)
      end
  end

categories_controller_test.rb

  context "on POST to :create " do
    setup { post :create, :category => { :name  => 'Cars', :description => 'I Like speed' } }
    should_set_the_flash_to "Category successfully added !"
  end