Handle different line item/inventory unit variants

core/app/models/spree/stock/availability_validator.rb

@@ -25,9 +25,13 @@ def is_valid?(line_item)
         if line_item.inventory_units.empty?
           Spree::Config.stock.quantifier_class.new(line_item.variant).can_supply?(line_item.quantity)
         else
-          quantity_by_stock_location_id = line_item.inventory_units.pending.joins(:shipment).group(:stock_location_id).count
-          quantity_by_stock_location_id.all? do |stock_location_id, quantity|
-            Spree::Config.stock.quantifier_class.new(line_item.variant, stock_location_id).can_supply?(quantity)
+          quantity_by_variant_and_stock_location = line_item.inventory_units.pending.joins(:shipment)
+            .group(:variant_id, :stock_location_id).count
+          variant_ids = quantity_by_variant_and_stock_location.keys.map(&:first)
+          variants = Spree::Variant.with_discarded.where(id: variant_ids).index_by(&:id)
+
+          quantity_by_variant_and_stock_location.all? do |(variant_id, stock_location_id), quantity|
+            Spree::Config.stock.quantifier_class.new(variants.fetch(variant_id), stock_location_id).can_supply?(quantity)
           end
         end
       end

core/spec/models/spree/stock/availability_validator_spec.rb

@@ -106,6 +106,42 @@ module Stock
             include_examples "passes validation"
           end
         end
+
+        context "when an inventory unit has a different variant than the line item" do
+          let(:line_item) { order.line_items.first }
+          let(:other_variant) { create(:variant) }
+
+          before do
+            create(:inventory_unit, variant: other_variant, line_item:, shipment: order.shipments.first)
+            line_item.variant.stock_items.update_all(count_on_hand: 10, backorderable: false)
+          end
+
+          context "and the other variant has no stock" do
+            before do
+              other_variant.stock_items.update_all(count_on_hand: 0, backorderable: false)
+            end
+
+            include_examples "fails validation"
+          end
+
+          context "and the other variant has stock" do
+            before do
+              other_variant.stock_items.update_all(count_on_hand: 1, backorderable: false)
+            end
+
+            include_examples "passes validation"
+          end
+
+          context "and only the other variant's units are pending" do
+            before do
+              line_item.inventory_units.where(variant: line_item.variant).update_all(pending: false)
+              line_item.variant.stock_items.update_all(count_on_hand: 0, backorderable: false)
+              other_variant.stock_items.update_all(count_on_hand: 1, backorderable: false)
+            end
+
+            include_examples "passes validation"
+          end
+        end
       end
 
       context "line_item is split across two shipments" do