Preload variant and product when calculating taxes to avoid an N+1

core/app/models/spree/tax_calculator/default.rb

@@ -23,6 +23,8 @@ def initialize(order)
       #
       # @return [Spree::Tax::OrderTax] the calculated taxes for the order
       def calculate
+        ActiveRecord::Associations::Preloader.new(records: order.line_items, associations: {variant: :product}).call
+
         Spree::Tax::OrderTax.new(
           order_id: order.id,
           order_taxes: order_rates,

core/spec/models/spree/tax_calculator/default_spec.rb

@@ -76,5 +76,18 @@
         expect(order_tax.label).to eq "Flat Book Fee"
       end
     end
+
+    context "with multiple line items" do
+      before do
+        2.times do
+          order.contents.add(FactoryBot.create(:product, tax_category: books_category).master)
+        end
+        order.reload
+      end
+
+      it "loads line item variants in a single query" do
+        expect { calculator.calculate }.to make_database_queries(matching: /from .spree_variants..*\bid. IN \(/im, count: 1)
+      end
+    end
   end
 end