Fixed N+1 adjustment loading in promotion recalculation

Recalculating promotions loaded each line item's and shipment's
adjustments with a separate query, so the adjustment query count grew
with the number of items in the order. This runs on every order
recalculation.

Preloading the adjustments for all items in one batch keeps the query
count constant regardless of order size. The same fix applies to both
the legacy and the new promotion adjusters.

Author: ikraamg

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