Coupon input in Spree cart

_form.html.erb

<!-- app/views/spree/orders/_form.html.erb -->
<%= order_form.text_field :coupon_code, :class => 'form-control', placeholder: Spree.t(:coupon_code) %>

orders_controller_decorator.rb

# app/controllers/spree/orders_controller_decorator.rb
module Spree
  OrdersController.class_eval do
    before_action :apply_coupon_code
    
    private

    def apply_coupon_code
      if params[:order] && params[:order][:coupon_code]
        @order.coupon_code = params[:order][:coupon_code]

        handler = PromotionHandler::Coupon.new(@order).apply

        if handler.error.present?
          flash.now[:error] = handler.error
          respond_with(@order) { |format| format.html { render :edit } } && return
        elsif handler.success
          flash[:success] = handler.success
        end
      end
    end
  end
end