Remove code that relates to Spree::ShippingRate#tax_rate

Since now we have the persisted Spree::ShippingRate#taxes, we
do not need to store one of the tax rates anymore on the shipping rate.

The data migration contains three lines of code from tax_rate.rb
so that the migration still runs in the future.

core/app/models/spree/shipping_rate.rb

@@ -2,7 +2,7 @@ module Spree
   class ShippingRate < Spree::Base
     belongs_to :shipment, class_name: 'Spree::Shipment'
     belongs_to :shipping_method, -> { with_deleted }, class_name: 'Spree::ShippingMethod', inverse_of: :shipping_rates
-    belongs_to :tax_rate, -> { with_deleted }, class_name: 'Spree::TaxRate'
+
     has_many :taxes,
              class_name: "Spree::ShippingRateTax",
              foreign_key: "shipping_rate_id",
@@ -18,10 +18,6 @@ class ShippingRate < Spree::Base
     extend DisplayMoney
     money_methods :amount
 
-    def calculate_tax_amount
-      tax_rate.compute_amount(self)
-    end
-
     def display_price
       price = display_amount.to_s
 
@@ -36,10 +32,6 @@ def display_price
     end
     alias_method :display_cost, :display_price
 
-    def display_tax_amount(tax_amount)
-      Spree::Money.new(tax_amount, currency: currency)
-    end
-
     private
 
     def tax_label_separator

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

@@ -33,22 +33,11 @@ def choose_default_shipping_rate(shipping_rates)
       def calculate_shipping_rates(package)
         shipping_methods(package).map do |shipping_method|
           cost = shipping_method.calculator.compute(package)
-          tax_category = shipping_method.tax_category
-          if tax_category
-            tax_rate = tax_category.tax_rates.detect do |rate|
-              # If the rate's zone matches the order's zone, a positive adjustment will be applied.
-              # If the rate is from the default tax zone, then a negative adjustment will be applied.
-              # See the tests in shipping_rate_spec.rb for an example of this.d
-              rate.zone == package.shipment.order.tax_zone || rate.zone.default_tax?
-            end
-          end
-
           if cost
             rate = shipping_method.shipping_rates.new(
               cost: cost,
               shipment: package.shipment
             )
-            rate.tax_rate = tax_rate if tax_rate
             Spree::Tax::ShippingRateTaxer.new.tax(rate)
           end
         end.compact

core/db/migrate/20160225152313_remove_tax_rate_from_shipping_rate.rb

@@ -0,0 +1,40 @@
+class RemoveTaxRateFromShippingRate < ActiveRecord::Migration
+  class Spree::ShippingRate < Spree::Base; end
+  class Spree::TaxRate < Spree::Base
+    has_one :calculator, class_name: "Spree::Calculator", as: :calculable, inverse_of: :calculable, dependent: :destroy, autosave: true
+    def compute_amount(item)
+      calculator.compute(item)
+    end
+  end
+
+  def up
+    say_with_time "Pre-calculating taxes for existing shipping rates" do
+      Spree::ShippingRate.find_each do |shipping_rate|
+        tax_rate_id = shipping_rate.tax_rate_id
+        if tax_rate_id
+          tax_rate = Spree::TaxRate.unscoped.find_by(shipping_rate.tax_rate_id)
+          shipping_rate.taxes.create(
+            tax_rate: tax_rate,
+            amount: tax_rate.compute_amount(shipping_rate)
+          )
+        end
+      end
+    end
+
+    remove_column :spree_shipping_rates, :tax_rate_id
+  end
+
+  def down
+    add_reference :spree_shipping_rates, :tax_rate, index: true, foreign_key: true
+    say_with_time "Attaching a tax rate to shipping rates" do
+      Spree::ShippingRate.find_each do |shipping_rate|
+        shipping_taxes = Spree::ShippingRateTax.where(shipping_rate_id: shipping_rate.id)
+        # We can only use one tax rate, so let's take the biggest.
+        selected_tax = shipping_taxes.sort_by(&:amount).last
+        if selected_tax
+          shipping_rate.update(tax_rate_id: tax_rate_id)
+        end
+      end
+    end
+  end
+end

core/spec/models/spree/shipping_rate_spec.rb

@@ -170,25 +170,6 @@
     end
   end
 
-  context "#tax_rate" do
-    let!(:tax_rate) { create(:tax_rate) }
-
-    before do
-      shipping_rate.tax_rate = tax_rate
-    end
-
-    it "can be retrieved" do
-      expect(shipping_rate.tax_rate.reload).to eq(tax_rate)
-    end
-
-    it "can be retrieved even when deleted" do
-      tax_rate.update_column(:deleted_at, Time.current)
-      shipping_rate.save
-      shipping_rate.reload
-      expect(shipping_rate.tax_rate).to eq(tax_rate)
-    end
-  end
-
   context "#shipping_method_code" do
     before do
       shipping_method.code = "THE_CODE"

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

@@ -148,7 +148,7 @@ module Stock
 
           it "links the shipping rate and the tax rate" do
             shipping_rates = subject.shipping_rates(package)
-            expect(shipping_rates.first.tax_rate).to eq(tax_rate)
+            expect(shipping_rates.first.taxes.first.tax_rate).to eq(tax_rate)
           end
         end