Extract and stub PaymentMethodFetcher

app/controllers/split_checkout_controller.rb

@@ -63,7 +63,7 @@ class SplitCheckoutController < ::BaseController
   end
 
   def selected_payment_method
-    @selected_payment_method ||= @order.payments.order(:created_at).last&.payment_method
+    @selected_payment_method ||= Checkout::PaymentMethodFetcher.new(@order).call
   end
 
   def update_order

app/services/checkout/payment_method_fetcher.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Checkout
+  class PaymentMethodFetcher
+    def initialize(order)
+      @order = order
+    end
+
+    def call
+      latest_payment&.payment_method
+    end
+
+    private
+
+    attr_reader :order
+
+    def latest_payment
+      order.payments.order(:created_at).last
+    end
+  end
+end

spec/controllers/split_checkout_controller_spec.rb

@@ -196,7 +196,8 @@ describe SplitCheckoutController, type: :controller do
 
       context "when an external payment gateway is used" do
         before do
-          expect(controller).to receive(:selected_payment_method).at_least(:once) { payment_method }
+          expect(Checkout::PaymentMethodFetcher).
+            to receive_message_chain(:new, :call) { payment_method }
           expect(payment_method).to receive(:external_gateway?) { true }
           expect(payment_method).to receive(:external_payment_url) { "https://example.com/pay" }
         end

spec/services/checkout/payment_method_fetcher_spec.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Checkout::PaymentMethodFetcher do
+  let!(:order) { create(:completed_order_with_totals) }
+  let(:payment1) { build(:payment, order: order) }
+  let(:payment2) { build(:payment, order: order) }
+  let(:service) { Checkout::PaymentMethodFetcher.new(order) }
+
+  describe '#call' do
+    context "when the order has payments" do
+      before do
+        order.payments << payment1
+        order.payments << payment2
+      end
+
+      it "returns the payment_method of the most recently created payment" do
+        expect(service.call).to eq payment2.payment_method
+      end
+    end
+
+    context "when the order has no payments" do
+      it "returns nil" do
+        expect(service.call).to be_nil
+      end
+    end
+  end
+end