Show last payment method in order confirmation

If we had multiple failed payments and then a successful payment, the
order confirmation was displaying the payment method of the first failed
payment. That was confusing and is now changed to the last payment
method.

app/helpers/orders_helper.rb

@@ -0,0 +1,10 @@
+module OrdersHelper
+  def order_paid_via(order)
+    # `sort_by` avoids additional database queries when payments are loaded
+    # already. There is usually only one payment and this shouldn't cause
+    # any overhead compared to `order(:updated_at)`.
+    #
+    # Using `last` without sort is not deterministic.
+    order.payments.sort_by(&:updated_at).last.andand.payment_method
+  end
+end

app/views/spree/shared/_order_details.html.haml

@@ -13,9 +13,9 @@
     .pad
       .text-big
         = t :order_payment
-        %strong=  order.payments.first.andand.payment_method.andand.name
+        %strong=  order_paid_via(order).andand.name
       %p.text-small.text-skinny.pre-line
-        %em= order.payments.first.andand.payment_method.andand.description
+        %em= order_paid_via(order).andand.description
 
     .order-summary.text-small
       %strong

spec/views/spree/shared/_order_details.html.haml_spec.rb

@@ -29,4 +29,26 @@ describe "spree/shared/_order_details.html.haml" do
 
     expect(rendered).to have_content("Paying via: Bar<script>evil</script>ter&rarr;ing")
   end
+
+  it "shows the last used payment method" do
+    first_payment = order.payments.first
+    second_payment = create(
+      :payment,
+      order: order,
+      payment_method: create(:payment_method, name: "Cash")
+    )
+    third_payment = create(
+      :payment,
+      order: order,
+      payment_method: create(:payment_method, name: "Credit")
+    )
+    first_payment.update_column(:updated_at, 3.days.ago)
+    second_payment.update_column(:updated_at, 2.days.ago)
+    third_payment.update_column(:updated_at, 1.day.ago)
+    order.payments.reload
+
+    render
+
+    expect(rendered).to have_content("Paying via: Credit")
+  end
 end