Command Sketch

add_item_to_cart_command.rb

class AddItemToCartCommand
  include Command

  validate :sku, presence: true
  validate :quantity, presence: true, numericality: { greater_than_or_equal_to: 1 }

  def cart
    Cart.from_session(params[:session_id])
  end

  def run
    cart.add_item(params[:sku], params[:quantity])
  end
end

carts_controller.rb

class CartsController < AppllicationController
  def add_item
    run_command :add_item_to_cart, params.merge(session_id: session_id) do
      success do
        flash[:success] = 'This item has been added to your cart.'
        redirect_to cart_path
      end

      failure do |errors|
        flash[:error] = errors
        redirect_to product_path(params[:id))
      end
    end
  end
end