Raise when a variant has more than a stock item

This means we violated the business rules of having a single stock
location for the OFN instance and hence a single stock item per variant.

Although it is too late to preserve the data integrity we can know data
needs to be cleaned up.

lib/open_food_network/variant_stock.rb

@@ -98,6 +98,7 @@ module OpenFoodNetwork
       warn_deprecation(__method__, 'Spree::Config[:track_inventory_levels]')
 
       raise_error_if_no_stock_item_available
+      raise_error_if_multiple_stock_items
 
       # There should be only one at the default stock location.
       #
@@ -125,6 +126,11 @@ module OpenFoodNetwork
       raise message if stock_items.empty?
     end
 
+    def raise_error_if_multiple_stock_items
+      message = 'A variant cannot have more than a stock item.'
+      raise message if stock_items.size > 1
+    end
+
     # Backwards compatible setting of stock levels in Spree 2.0.
     # It would be better to use `Spree::StockItem.adjust_count_on_hand` which
     # takes a value to add to the current stock level and uses proper locking.

spec/lib/open_food_network/variant_stock_spec.rb

@@ -129,6 +129,21 @@ describe OpenFoodNetwork::VariantStock do
   end
 
   describe '#on_demand=' do
+    context 'when the variant has multiple stock items' do
+      let(:variant) { create(:variant) }
+
+      before do
+        # Spree creates a stock_item for each variant when creating a stock
+        # location by means of #propagate_variant
+        create(:stock_location, name: 'location')
+        create(:stock_location, name: 'another location')
+      end
+
+      it 'raises' do
+        expect { variant.on_demand = true }.to raise_error(StandardError)
+      end
+    end
+
     context 'when the variant has a stock item' do
       let(:variant) { create(:variant, on_demand: true) }