Preload variants when building inventory units to avoid an N+1

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

@@ -8,6 +8,7 @@ def initialize(order)
       end
 
       def units
+        ActiveRecord::Associations::Preloader.new(records: @order.line_items, associations: {variant: :product}).call
         @order.line_items.flat_map do |line_item|
           build_units(line_item, line_item.quantity)
         end

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

@@ -28,6 +28,16 @@ module Stock
         it "builds the inventory units as pending" do
           expect(subject.units.map(&:pending).uniq).to eq [true]
         end
+
+        context "with multiple line items" do
+          let(:order) { create(:order_with_line_items, line_items_count: 3) }
+
+          before { order.reload }
+
+          it "loads line item variants in a single query" do
+            expect { described_class.new(order).units }.to make_database_queries(matching: /from .spree_variants..*\bid. IN \(/im, count: 1)
+          end
+        end
       end
 
       describe "#missing_units_for_line_item" do