Preload adjustments in promotion recalculation to avoid an N+1

legacy_promotions/app/models/spree/promotion/order_adjustments_recalculator.rb

@@ -15,6 +15,7 @@ def initialize(order)
 
       def call(persist: true)
         all_items = line_items + shipments
+        ActiveRecord::Associations::Preloader.new(records: all_items, associations: :adjustments).call
         all_items.each do |item|
           promotion_adjustments = item.adjustments.select(&:promotion?)
 

legacy_promotions/spec/models/spree/promotion/order_adjustments_recalculator_spec.rb

@@ -18,6 +18,16 @@ def initialize(_adjustments)
       let(:order) { create(:order_with_line_items, line_items_count: 1, line_items_price: 10) }
       let(:line_item) { order.line_items[0] }
 
+      context "with multiple line items" do
+        let(:order) { create(:order_with_line_items, line_items_count: 3) }
+
+        before { order.reload }
+
+        it "loads line item adjustments in a single query" do
+          expect { subject }.to make_database_queries(matching: /from .spree_adjustments..*adjustable_id. IN \(/im, count: 1)
+        end
+      end
+
       context "when the quantity changes with a CreateQuantityAdjustments promotion" do
         let(:promotion) { create(:promotion, promotion_actions: [promotion_action]) }
         let(:promotion_action) { Spree::Promotion::Actions::CreateQuantityAdjustments.new(calculator:, preferred_group_size: 2) }

promotions/app/models/solidus_promotions/order_adjuster.rb

@@ -22,6 +22,8 @@ def initialize(order, dry_run_promotion: nil)
     def call(persist: true) # rubocop:disable Lint/UnusedMethodArgument
       return order unless SolidusPromotions::Promotion.order_activatable?(order)
 
+      ActiveRecord::Associations::Preloader.new(records: order.line_items + order.shipments, associations: :adjustments).call
+
       SetDiscountsToZero.call(order)
 
       DiscountOrder.new(order, promotions, dry_run: dry_run).call

promotions/spec/models/solidus_promotions/order_adjuster_spec.rb

@@ -237,4 +237,21 @@
       end
     end
   end
+
+  context "with multiple line items" do
+    let(:order) { create(:order_with_line_items, line_items_count: 3) }
+    let!(:benefit) do
+      SolidusPromotions::Benefits::AdjustLineItem.create(promotion: promotion, calculator: calculator)
+    end
+
+    before do
+      order_adjuster.call
+      order.save!
+      order.reload
+    end
+
+    it "loads line item adjustments in a single query" do
+      expect { order_adjuster.call }.to make_database_queries(matching: /from .spree_adjustments..*adjustable_id. IN \(/im, count: 1)
+    end
+  end
 end