Add discounts to items after calculation

Otherwise we get discounted amounts from discounts from the current lane
(which we don't want).

promotions/app/models/solidus_friendly_promotions/friendly_promotion_discounter.rb

@@ -15,9 +15,12 @@ def call
       SolidusFriendlyPromotions::Promotion.ordered_lanes.each do |lane, _index|
         lane_promotions = promotions.select { |promotion| promotion.lane == lane }
         item_discounter = ItemDiscounter.new(promotions: lane_promotions)
-        adjust_line_items(item_discounter)
-        adjust_shipments(item_discounter)
-        adjust_shipping_rates(item_discounter)
+        line_item_discounts = adjust_line_items(item_discounter)
+        shipment_discounts = adjust_shipments(item_discounter)
+        shipping_rate_discounts = adjust_shipping_rates(item_discounter)
+        (line_item_discounts + shipment_discounts + shipping_rate_discounts).each do |item, chosen_discounts|
+          item.discounts.concat(chosen_discounts)
+        end
       end
 
       order
@@ -28,26 +31,26 @@ def call
     def adjust_line_items(item_discounter)
       order.line_items.select do |line_item|
         line_item.variant.product.promotionable?
-      end.flat_map do |line_item|
+      end.map do |line_item|
         discounts = item_discounter.call(line_item)
         chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(line_item).call(discounts)
-        line_item.discounts.concat(chosen_item_discounts)
+        [line_item, chosen_item_discounts]
       end
     end
 
     def adjust_shipments(item_discounter)
-      order.shipments.flat_map do |shipment|
+      order.shipments.map do |shipment|
         discounts = item_discounter.call(shipment)
         chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(shipment).call(discounts)
-        shipment.discounts.concat(chosen_item_discounts)
+        [shipment, chosen_item_discounts]
       end
     end
 
     def adjust_shipping_rates(item_discounter)
-      order.shipments.flat_map(&:shipping_rates).flat_map do |rate|
+      order.shipments.flat_map(&:shipping_rates).map do |rate|
         discounts = item_discounter.call(rate)
         chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(rate).call(discounts)
-        rate.discounts.concat(chosen_item_discounts)
+        [rate, chosen_item_discounts]
       end
     end
 

promotions/spec/models/promotion/integration_spec.rb

@@ -51,7 +51,7 @@
     end
   end
 
-  context "with two promotions that should stack", :pending do
+  context "with two promotions that should stack" do
     let(:shirt) { create(:product, name: "Shirt", price: 30) }
     let(:pants) { create(:product, name: "Pants", price: 40) }