Update order state when moving back through checkout step

Some important logic happens after the order transition from one state
to another. In particular, voucher are recalculated. To fix any
inconsistency, we make sure the order is the state matching the checkout
step we are on.

app/controllers/concerns/checkout_steps.rb

@@ -13,6 +13,10 @@ module CheckoutSteps
     params[:step] == "payment"
   end
 
+  def details_step?
+    params[:step] == "details"
+  end
+
   def redirect_to_step_based_on_order
     case @order.state
     when "cart", "address", "delivery"
@@ -39,9 +43,19 @@ module CheckoutSteps
   def check_step
     case @order.state
     when "cart", "address", "delivery"
-      redirect_to checkout_step_path(:details) unless params[:step] == "details"
+      redirect_to checkout_step_path(:details) unless details_step?
     when "payment"
-      redirect_to checkout_step_path(:payment) if params[:step] == "summary"
+      redirect_to checkout_step_path(:payment) if summary_step?
     end
   end
+
+  def update_order_state
+    if @order.state == "confirmation" && payment_step?
+      @order.back_to_payment
+    end
+
+    return unless @order.state.in?(["payment", "confirmation"]) && details_step?
+
+    @order.back_to_address
+  end
 end

app/controllers/split_checkout_controller.rb

@@ -24,6 +24,8 @@ class SplitCheckoutController < ::BaseController
 
   def edit
     redirect_to_step_based_on_order unless params[:step]
+
+    update_order_state if params[:step]
     check_step if params[:step]
 
     return if available_shipping_methods.any?

spec/controllers/split_checkout_controller_spec.rb

@@ -67,6 +67,42 @@ describe SplitCheckoutController, type: :controller do
           expect(response).to redirect_to checkout_step_path(:payment)
         end
       end
+
+      context "when order state is 'confirmation'" do
+        before do
+          order.update!(state: "confirmation")
+        end
+
+        context "when loading payment step" do
+          it "updates the order state to payment" do
+            get :edit, params: { step: "payment" }
+
+            expect(order.reload.state).to eq("payment")
+          end
+        end
+
+        context "when loading address step" do
+          it "updates the order state to address" do
+            get :edit, params: { step: "details" }
+
+            expect(order.reload.state).to eq("address")
+          end
+        end
+      end
+
+      context "when order state is 'payment'" do
+        context "when loading address step" do
+          before do
+            order.update!(state: "payment")
+          end
+
+          it "updates the order state to address" do
+            get :edit, params: { step: "details" }
+
+            expect(order.reload.state).to eq("address")
+          end
+        end
+      end
     end
   end