Fixed N+1 loading tax categories during tax calculation

Calculating order taxes read each line item's tax category through its
variant and, when the variant had none, through its product. With the
associations unloaded that meant two queries per line item, every time
taxes were recalculated.

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

Author: ikraamg

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".*id" IN \(/im, count: 1)
+      end
+    end
   end
 end