Preload variants when checking order promotionability

Checking whether an order contains a non-promotionable product loaded
each line item's variant and product with a query per line item. This
runs every time a promotion's eligibility is evaluated.

Preloading the variant and product for all line items keeps that load
constant regardless of how many items the order has.

Author: ikraamg

legacy_promotions/app/models/spree/promotion.rb

@@ -240,6 +240,7 @@ def blacklisted?(promotable)
       when Spree::LineItem
         !promotable.variant.product.promotionable?
       when Spree::Order
+        ActiveRecord::Associations::Preloader.new(records: promotable.line_items, associations: {variant: :product}).call
         promotable.line_items.any? { |line_item| !line_item.variant.product.promotionable? }
       end
     end

legacy_promotions/spec/models/spree/promotion_spec.rb

@@ -714,6 +714,17 @@
           it { is_expected.to be true }
         end
       end
+
+      context "with several line items" do
+        let(:promotion) { create(:promotion, :with_line_item_adjustment, apply_automatically: true) }
+        let(:promotable) { create(:order_with_line_items, line_items_count: 3) }
+
+        before { promotable.reload }
+
+        it "loads line item variants in a single query" do
+          expect { subject }.to make_database_queries(matching: /from .spree_variants..*\bid. IN \(/im, count: 1)
+        end
+      end
     end
 
     context "when promotable is a Spree::LineItem" do