order_placing_service.rb

class OrderPlacingService
  def initialize(cart,order)
    @order = order
    @cart = cart
  end

  def place_order!
    @order.build_item_cache_from_cart(@cart)
    @order.calculate_total!(@cart)
    @cart.clear!
    OrderMailer.notify_order_placed(@order).deliver
  end
end