Require StandingOrders to have at least one product (StandingLineItem)

app/forms/standing_order_form.rb

@@ -18,6 +18,7 @@ class StandingOrderForm
   validate :schedule_allowed?
   validate :payment_method_allowed?
   validate :shipping_method_allowed?
+  validate :standing_line_items_present?
   validate :standing_line_items_available?
 
   def initialize(standing_order, params={}, fee_calculator=nil)
@@ -209,6 +210,12 @@ class StandingOrderForm
     end
   end
 
+  def standing_line_items_present?
+    unless standing_line_items.reject(&:marked_for_destruction?).any?
+      errors.add(:standing_line_items, :at_least_one_product)
+    end
+  end
+
   def standing_line_items_available?
     standing_line_items.each do |sli|
       unless sli.available_from?(shop_id, schedule_id)

config/locales/en.yml

@@ -76,6 +76,7 @@ en:
         standing_order_form:
           attributes:
             standing_line_items:
+              at_least_one_product: "^Please add at least one product"
               not_available: "^%{name} is not available from the selected schedule"
   devise:
     confirmations:

spec/forms/standing_order_form_spec.rb

@@ -384,12 +384,32 @@ describe StandingOrderForm do
     let(:params) { { standing_line_items_attributes: [ { id: sli.id, _destroy: true } ] } }
     let(:form) { StandingOrderForm.new(standing_order, params) }
 
-    it "removes the line item and updates totals on all orders" do
-      expect(order.reload.total.to_f).to eq 59.97
-      expect(form.save).to be true
-      line_items = Spree::LineItem.where(order_id: standing_order.orders, variant_id: variant.id)
-      expect(line_items.count).to be 0
-      expect(order.reload.total.to_f).to eq 39.98
+    context "that is not the last remaining item" do
+      it "removes the line item and updates totals on all orders" do
+        expect(order.reload.total.to_f).to eq 59.97
+        expect(form.save).to be true
+        line_items = Spree::LineItem.where(order_id: standing_order.orders, variant_id: variant.id)
+        expect(line_items.count).to be 0
+        expect(order.reload.total.to_f).to eq 39.98
+      end
+    end
+
+    context "that is the last remaining item" do
+      before do
+        standing_order.standing_line_items.where('variant_id != ?',variant.id).destroy_all
+        order.line_items.where('variant_id != ?',variant.id).destroy_all
+        standing_order.reload
+        order.reload
+      end
+
+      it "returns false and does not remove the line item or update totals on orders" do
+        expect(order.reload.total.to_f).to eq 19.99
+        expect(form.save).to be false
+        line_items = Spree::LineItem.where(order_id: standing_order.orders, variant_id: variant.id)
+        expect(line_items.count).to be 1
+        expect(order.reload.total.to_f).to eq 19.99
+        expect(form.json_errors.keys).to eq [:standing_line_items]
+      end
     end
   end