Add FreeShipping#discount

core/app/models/spree/promotion/actions/free_shipping.rb

@@ -4,6 +4,19 @@ module Spree
   class Promotion < Spree::Base
     module Actions
       class FreeShipping < Spree::PromotionAction
+        def discountable_class
+          Spree::Shipment
+        end
+
+        def discount(shipment)
+          discount = shipment.discounts.detect do |discount|
+            discount.promotion_action == self
+          end || shipment.discounts.build(promotion_action: self)
+          discount.label = I18n.t('spree.adjustment_labels.shipment', promotion: Spree::Promotion.model_name.human, promotion_name: promotion.name)
+          discount.amount = compute_amount(shipment)
+          discount
+        end
+
         def perform(payload = {})
           order = payload[:order]
           promotion_code = payload[:promotion_code]

core/config/locales/en.yml

@@ -825,6 +825,7 @@ en:
     adjustment_labels:
       line_item: "%{promotion} (%{promotion_name})"
       order: "%{promotion} (%{promotion_name})"
+      shipment: "%{promotion} (%{promotion_name})"
       tax_rates:
         sales_tax: "%{name}"
         sales_tax_with_rate: "%{name} %{amount}"

core/spec/models/spree/promotion/actions/free_shipping_spec.rb

@@ -10,6 +10,32 @@
   let(:payload) { { order: order, promotion_code: promotion_code } }
   let(:promotion_code) { promotion.codes.first! }
 
+  context "#discount" do
+    subject { action.discount(shipment) }
+
+    before do
+      expect(action).to receive(:compute_amount).and_return(10)
+      action.promotion = promotion
+    end
+
+    it "builds an unpersisted discount" do
+      expect(subject).to be_a(Spree::ShipmentDiscount)
+      expect(subject.amount).to eq(10)
+      expect(subject.label).to eq("Promotion (Promo)")
+    end
+
+    context "if the shipment already has a discount" do
+      let!(:existing_shipment_discount) { create(:shipment_discount, shipment: shipment, promotion_action: action) }
+
+      it "changes the discounts label and price" do
+        expect(subject).to be_a(Spree::ShipmentDiscount)
+        expect(subject.amount).to eq(10)
+        expect(subject.label).to eq("Promotion (Promo)")
+        expect(subject).to be_persisted
+        expect(subject).to be_changed
+      end
+    end
+  end
   # From promotion spec:
   describe "#perform" do
     before do