Add specs to cover re calculation

It is important that the calculated voucher adjustments don't change
if they are recalculated and it is equally important that they are
updated if the order has changed

spec/services/voucher_adjustments_service_spec.rb

@@ -56,6 +56,38 @@ describe VoucherAdjustmentsService do
         # -0.0625 * 10 = -0.625
         expect(subject.included_tax.to_f).to eq(-0.63)
       end
+
+      context "when re calculating" do
+        it "does not update the adjustment amount" do
+          expect do
+            VoucherAdjustmentsService.calculate(order)
+          end.not_to change { subject.reload.amount }
+        end
+
+        it "does not update the tax adjustment" do
+          expect do
+            VoucherAdjustmentsService.calculate(order)
+          end.not_to change { subject.reload.included_tax }
+        end
+
+        context "when the order changed" do
+          before do
+            order.update_columns(item_total: 200)
+          end
+
+          it "does update the adjustment amount" do
+            expect do
+              VoucherAdjustmentsService.calculate(order)
+            end.not_to change { subject.reload.amount }
+          end
+
+          it "updates the tax adjustment" do
+            expect do
+              VoucherAdjustmentsService.calculate(order)
+            end.to change { subject.reload.included_tax }
+          end
+        end
+      end
     end
 
     context 'with tax not included in order price' do
@@ -70,6 +102,8 @@ describe VoucherAdjustmentsService do
           tax_rate_name: "Tax 1"
         )
       end
+      let(:adjustment) { order.voucher_adjustments.first }
+      let(:tax_adjustment) { order.voucher_adjustments.second }
 
       before do
         # create adjustment before tax are set
@@ -84,7 +118,6 @@ describe VoucherAdjustmentsService do
       end
 
       it 'includes amount without tax' do
-        adjustment = order.voucher_adjustments.first
         # voucher_rate = amount / order.total
         # -10 / 171 = -0.058479532
         # amount = voucher_rate * (order.total - order.additional_tax_total)
@@ -97,19 +130,39 @@ describe VoucherAdjustmentsService do
         # -10 / 171 = -0.058479532
         # amount = voucher_rate * order.additional_tax_total
         # -0.058479532 * 11 = -0.64
-        tax_adjustment = order.voucher_adjustments.second
         expect(tax_adjustment.amount.to_f).to eq(-0.64)
         expect(tax_adjustment.label).to match("Tax")
       end
 
       context "when re calculating" do
-        it "updates the tax adjustment" do
-          order.update_columns(item_total: 200)
-          tax_adjustment = order.voucher_adjustments.second
-
+        it "does not update the adjustment amount" do
           expect do
             VoucherAdjustmentsService.calculate(order)
-          end.to change { tax_adjustment.reload.amount }
+          end.not_to change { adjustment.reload.amount }
+        end
+
+        it "does not update the tax adjustment" do
+          expect do
+            VoucherAdjustmentsService.calculate(order)
+          end.not_to change { tax_adjustment.reload.amount }
+        end
+
+        context "when the order changed" do
+          before do
+            order.update_columns(item_total: 200)
+          end
+
+          it "updates the adjustment amount" do
+            expect do
+              VoucherAdjustmentsService.calculate(order)
+            end.to change { adjustment.reload.amount }
+          end
+
+          it "updates the tax adjustment" do
+            expect do
+              VoucherAdjustmentsService.calculate(order)
+            end.to change { tax_adjustment.reload.amount }
+          end
         end
       end
     end