Speed up taxation of shipping rates

Instead of loading the tax rates for the order for each rate which needs
taxing, we can perform the SQL queries only once.

This is done by building a single instance of the
shipping_rate_tax_calculator_class for a shipment, and calling
`calculate` on that object for each rate, allowing it to cache the order
rates.

core/app/models/spree/stock/estimator.rb

@@ -31,14 +31,22 @@ def choose_default_shipping_rate(shipping_rates)
       end
 
       def calculate_shipping_rates(package)
+        tax_calculator_class = Spree::Config.shipping_rate_tax_calculator_class
+        tax_calculator = tax_calculator_class.new(package.shipment.order)
         shipping_methods(package).map do |shipping_method|
           cost = shipping_method.calculator.compute(package)
           if cost
             rate = shipping_method.shipping_rates.new(
               cost: cost,
               shipment: package.shipment
             )
-            Spree::Config.shipping_rate_taxer_class.new.tax(rate)
+            tax_calculator.calculate(rate).each do |tax|
+              rate.taxes.new(
+                amount: tax.amount,
+                tax_rate: tax.tax_rate
+              )
+            end
+            rate
           end
         end.compact
       end

core/app/models/spree/tax/shipping_rate_taxer.rb

@@ -8,7 +8,7 @@ class ShippingRateTaxer
       #   This parameter will be modified.
       # @return [Spree::ShippingRate] The shipping rate with associated tax objects
       def tax(shipping_rate)
-        taxes = Spree::Config.shipping_rate_tax_calculator_class.new(shipping_rate).calculate
+        taxes = Spree::Config.shipping_rate_tax_calculator_class.new(shipping_rate.order).calculate(shipping_rate)
         taxes.each do |tax|
           shipping_rate.taxes.build(
             amount: tax.amount,

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

@@ -21,15 +21,24 @@ class ShippingRate
       # @param [Spree::ShippingRate] shipping_rate the shipping rate to
       #   calculate taxes on
       # @return [Spree::TaxCalculator::ShippingRate]
-      def initialize(shipping_rate)
-        @shipping_rate = shipping_rate
+      def initialize(order)
+        if order.is_a?(::Spree::ShippingRate)
+          Spree::Deprecation.warn "passing a single shipping rate to Spree::TaxCalculator::ShippingRate is DEPRECATED. It now expects an order"
+          shipping_rate = order
+          @order = shipping_rate.order
+          @shipping_rate = shipping_rate
+        else
+          @order = order
+          @shipping_rate = nil
+        end
       end
 
       # Calculate taxes for a shipping rate.
       #
       # @return [Array<Spree::Tax::ItemTax>] the calculated taxes for the
       #   shipping rate
-      def calculate
+      def calculate(shipping_rate)
+        shipping_rate ||= @shipping_rate
         rates_for_item(shipping_rate).map do |rate|
           amount = rate.compute_amount(shipping_rate)
 

core/lib/spree/app_configuration.rb

@@ -287,8 +287,6 @@ def default_pricing_options
 
     class_name_attribute :shipping_rate_selector_class, default: 'Spree::Stock::ShippingRateSelector'
 
-    class_name_attribute :shipping_rate_taxer_class, default: 'Spree::Tax::ShippingRateTaxer'
-
     # Allows providing your own class for calculating taxes on a shipping rate.
     #
     # @!attribute [rw] shipping_rate_tax_calculator_class

core/spec/models/spree/stock/estimator_spec.rb

@@ -189,20 +189,19 @@ class Spree::Stock::TestSorter; end;
         end
 
         it 'uses the configured shipping rate taxer' do
-          class Spree::Tax::TestTaxer
-            def initialize
+          class Spree::Tax::TestTaxCalculator
+            def initialize(_order)
             end
 
-            def tax(_)
-              Spree::ShippingRate.new
+            def calculate(_shipping_rate)
+              [
+                Spree::Tax::ItemTax.new(label: "TAX", amount: 5)
+              ]
             end
           end
-          Spree::Config.shipping_rate_taxer_class = Spree::Tax::TestTaxer
-
-          shipping_rate = Spree::ShippingRate.new
-          allow(Spree::ShippingRate).to receive(:new).and_return(shipping_rate)
+          Spree::Config.shipping_rate_tax_calculator_class = Spree::Tax::TestTaxCalculator
 
-          expect(Spree::Tax::TestTaxer).to receive(:new).and_call_original
+          expect(Spree::Tax::TestTaxCalculator).to receive(:new).and_call_original
           subject.shipping_rates(package)
         end
       end