orders_controller_decorator.rb

  # Adds a new item to the order (creating a new order if none already exists)
  def populate
    ap "#populate"
    ap request
    # clear cart for buy now action if necessary
    ap params
    ap @current_order if @current_order
    @current_order.line_items.destroy_all if @current_order && (params[:order] and params[:order][:buy_now])

    # populate an order
    populator = Spree::OrderPopulator.new(current_order(true), current_currency)
    if populator.populate(params.slice(:products, :variants, :quantity))
      fire_event('spree.cart.add')
      fire_event('spree.order.contents_changed')
      if !(params[:order] and params[:order][:buy_now])
        # add to cart
        respond_with(@order) do |format|
          format.html { redirect_to cart_path }
        end
      else
        # buy now
        respond_with(@order) do |format|
          format.html { redirect_to checkout_state_path(populator.order.state) }
        end
      end
    else
      flash[:error] = populator.errors.full_messages.join(" ")
      redirect_to :back
    end
  end

orders_controller_spec.rb

require 'spec_helper'

describe Spree::OrdersController do
  let(:user) { create(:user) }
  let(:big_order)   { create :order_with_line_items, line_items_count: 5 }
  let(:small_order) { create :order_with_line_items, line_items_count: 1 }

  context "#populate" do
    # def spree_post(action, parameters = nil, session = nil, flash = nil)
    context "buy now" do
      it "should redirect_to spree.checkout_state_path if params[:order] and params[:order][:buy_now] are present" do
        small_order.state = "address"
        controller.instance_variable_set(:@current_order, small_order)
        spree_post :populate, { :order => { :buy_now => "true" } }
        response.should redirect_to spree.checkout_state_path(small_order.state)
      end
    end
  end
end