Adapt line_item_syncer to the case where item is not added to the completed order because of insufficient stock and the subscription quantity is updated for that item afterwards

app/services/line_item_syncer.rb

@@ -22,9 +22,15 @@ class LineItemSyncer
   def update_item_quantities(order)
     changed_subscription_line_items.each do |sli|
       line_item = order.line_items.find_by_variant_id(sli.variant_id)
-      next if update_quantity(line_item, sli)
 
-      add_order_update_issue(order, line_item)
+      if line_item.blank?
+        order_update_issues.add(order, sli.variant.product_and_full_name)
+        next
+      end
+
+      unless update_quantity(line_item, sli)
+        add_order_update_issue(order, line_item)
+      end
     end
   end
 

spec/services/order_syncer_spec.rb

@@ -457,8 +457,10 @@ describe OrderSyncer do
 
         it "does not change the quantity, and adds the order to order_update_issues" do
           expect(order.reload.total.to_f).to eq 79.96
+
           subscription.assign_attributes(params)
           expect(syncer.sync!).to be true
+
           expect(changed_line_item.reload.quantity).to eq 2
           expect(order.reload.total.to_f).to eq 79.96
           expect(syncer.order_update_issues[order.id]).to include "#{changed_line_item.product.name} - #{changed_line_item.variant.full_name}"
@@ -514,6 +516,24 @@ describe OrderSyncer do
           expect(order.reload.total.to_f).to eq 59.97
           expect(syncer.order_update_issues[order.id]).to include "#{variant.product.name} - #{variant.full_name} - Insufficient stock available"
         end
+
+        context "and then updating the quantity of that subscription line item that was not added to the completed order" do
+          it "does nothing to the order and adds the order to order_update_issues" do
+            expect(syncer.sync!).to be true
+
+            line_items = Spree::LineItem.where(order_id: subscription.orders, variant_id: variant.id)
+            expect(line_items.map(&:quantity)).to eq []
+
+            subscription.save # this is necessary to get an id on the subscription_line_items
+            params =  { subscription_line_items_attributes: [{ id: subscription.subscription_line_items.last.id, quantity: 2 }] }
+            subscription.assign_attributes(params)
+            expect(syncer.sync!).to be true
+
+            line_items = Spree::LineItem.where(order_id: subscription.orders, variant_id: variant.id)
+            expect(line_items.map(&:quantity)).to eq []
+            expect(syncer.order_update_issues[order.id]).to include "#{variant.product.name} - #{variant.full_name}"
+          end
+        end
       end
     end
   end