Fix cart disappearing bug after adding a voucher

The split checkout page uses `mrujs` and `CableCar` to set the form as
a remote one and perform `CableCar` operation if any : https://mrujs.com/how-tos/integrate-cablecar
The previous solution broke the cart handled by angularJS, it looks like
`morpdom` (https://mrujs.com/references/remote-forms-and-links Navigation Adapter section )
was doing something that angularJS didn't like when redirecting to
the current page.
With the power of `CableCar`, we now replace the #voucher-section on the
split checkout page instead of doing a redirect.

app/controllers/split_checkout_controller.rb

@@ -31,7 +31,7 @@ class SplitCheckoutController < ::BaseController
 
   def update
     if add_voucher
-      return redirect_to checkout_step_path(:payment)
+      return render_voucher_section
     elsif @order.errors.present?
       return render_error
     end
@@ -56,7 +56,7 @@ class SplitCheckoutController < ::BaseController
     adjustment = Spree::Adjustment.find_by(id: params[:adjustment_id])
     adjustment.destroy
 
-    redirect_to checkout_step_path(:payment)
+    render_voucher_section
   end
 
   private
@@ -72,6 +72,20 @@ class SplitCheckoutController < ::BaseController
       replace("#flashes", partial("shared/flashes", locals: { flashes: flash }))
   end
 
+  # Using the power of cable_car we replace only the #voucher_section instead of reloading the page
+  def render_voucher_section
+    render(
+      status: :ok,
+      operations: cable_car.replace(
+        "#voucher-section",
+        partial(
+          "split_checkout/voucher_section",
+          locals: { order: @order, voucher_adjustment: @order.vouchers.first }
+        )
+      )
+    )
+  end
+
   def order_error_messages
     # Remove ship_address.* errors if no shipping method is not selected
     remove_ship_address_errors if no_ship_address_needed?
@@ -193,7 +207,7 @@ class SplitCheckoutController < ::BaseController
   end
 
   def add_voucher
-    return unless payment_step? && params[:order] && params[:order][:voucher_code]
+    return unless payment_step? && params[:order] && params[:order][:voucher_code].present?
 
     # Fetch Voucher
     voucher = Voucher.find_by(code: params[:order][:voucher_code], enterprise: @order.distributor)

app/views/split_checkout/_payment.html.haml

@@ -1,24 +1,6 @@
 .medium-6
   %div.checkout-substep{"data-controller": "paymentmethod"}
-    - if @order.distributor.vouchers.present?
-      .checkout-title
-        = t("split_checkout.step2.voucher.apply_voucher")
-
-      .checkout-input
-        .two-columns-inputs.voucher
-          - if @voucher_adjustment.present?
-            %span.button.voucher-added
-              %i.ofn-i_051-check-big
-                = "#{@voucher_adjustment.originator.display_value} #{t("split_checkout.step2.voucher.voucher")}"
-            = link_to t("split_checkout.step2.voucher.remove_code"), checkout_destroy_path(adjustment_id: @voucher_adjustment.id), method: "delete", data: { confirm: t("split_checkout.step2.voucher.confirm_delete") }
-            - if @voucher_adjustment.originator.value > @order.total
-              .checkout-input
-                %span.formError.standalone
-                  = t("split_checkout.step2.voucher.warning_forfeit_remaining_amount")
-          - else
-            = f.text_field :voucher_code, { placeholder: t("split_checkout.step2.voucher.placeholder"), class: "voucher" }
-            - # TODO: enable button when code entered
-            = f.submit t("split_checkout.step2.voucher.apply"), class: "button cancel voucher", disabled: false 
+    = render "split_checkout/voucher_section", { order: @order, voucher_adjustment: @voucher_adjustment }
         
     %div.checkout-title
       = t("split_checkout.step2.payment_method.title")

app/views/split_checkout/_voucher_section.html.haml

@@ -0,0 +1,22 @@
+%div#voucher-section
+  = form_with url: checkout_update_path(:payment), model: order, method: :put, data: { remote: "true" } do |f|
+    - if order.distributor.vouchers.present?
+      .checkout-title
+        = t("split_checkout.step2.voucher.apply_voucher")
+
+      .checkout-input
+        .two-columns-inputs.voucher
+          - if voucher_adjustment.present?
+            %span.button.voucher-added
+              %i.ofn-i_051-check-big
+                = "#{voucher_adjustment.originator.display_value} #{t("split_checkout.step2.voucher.voucher")}"
+            = link_to t("split_checkout.step2.voucher.remove_code"), checkout_destroy_path(adjustment_id: voucher_adjustment.id), method: "delete", data: { confirm: t("split_checkout.step2.voucher.confirm_delete") }
+            - # TODO: this might not be true, ie payment method include fee which wouldn't be covered by voucher, tax implication raise total to be bigger than voucher other ?
+            - if voucher_adjustment.originator.value > order.total
+              .checkout-input
+                %span.formError.standalone
+                  = t("split_checkout.step2.voucher.warning_forfeit_remaining_amount")
+          - else
+            = f.text_field :voucher_code, { placeholder: t("split_checkout.step2.voucher.placeholder"), class: "voucher" }
+            - # TODO: enable button when code entered
+            = f.submit t("split_checkout.step2.voucher.apply"), class: "button cancel voucher", disabled: false 

spec/controllers/split_checkout_controller_spec.rb

@@ -251,7 +251,7 @@ describe SplitCheckoutController, type: :controller do
           it "adds a voucher to the order" do
             put :update, params: params
 
-            expect(response).to redirect_to checkout_step_path(:payment)
+            expect(response.status).to eq(200)
             expect(order.reload.vouchers.length).to eq(1)
           end
 
@@ -292,7 +292,7 @@ describe SplitCheckoutController, type: :controller do
 
             delete :destroy, params: { adjustment_id: adjustment.id }
 
-            expect(response).to redirect_to checkout_step_path(:payment)
+            expect(response.status).to eq(200)
             expect(order.reload.vouchers.length).to eq(0)
           end
         end