Detect changes in price

Price is actually an association with lots of custom methods to make it look like a field, and so changes were ignored.
Now this issue is fixed, perhaps it should be moved to a concern..

Note, there are other delegated fields: product name and description may be assigned from the variant. But there's no hooks to save the prroduct, so I didn't include it when checking for changes.

app/models/spree/variant.rb

@@ -199,6 +199,11 @@ module Spree
       price_in(currency).try(:amount)
     end
 
+    def changed?
+      # Changes to price are saved after_save
+      super || default_price.changed?
+    end
+
     # can_supply? is implemented in VariantStock
     def in_stock?(quantity = 1)
       can_supply?(quantity)

spec/models/spree/variant_spec.rb

@@ -58,6 +58,28 @@ RSpec.describe Spree::Variant do
     end
   end
 
+  describe "#changed?" do
+    subject(:variant) { create(:variant) }
+
+    it { is_expected.not_to be_changed }
+
+    it "is changed when basic fields are changed" do
+      subject.display_name = "blah"
+      expect(subject).to be_changed
+    end
+
+    describe "default_price" do
+      it "price" do
+        subject.price = 100
+        expect(subject).to be_changed
+      end
+      it "currency" do
+        subject.currency = "USD"
+        expect(subject).to be_changed
+      end
+    end
+  end
+
   context "price parsing" do
     context "price=" do
       context "with decimal point" do