Move tax rate finding into service and spec it

The `#tax_rates` method is actually not present in Spree. We encapsulate
the logic in this commit and enable better unit testing. The new tests
cover a current bug: https://github.com/openfoodfoundation/openfoodnetwork/issues/3127

app/models/spree/adjustment_decorator.rb

@@ -42,29 +42,7 @@ module Spree
 
     # @return [Array<Spree::TaxRate>]
     def tax_rates
-      case originator
-      when Spree::TaxRate
-        [originator]
-      when EnterpriseFee
-        case source
-        when Spree::LineItem
-          tax_category = originator.inherits_tax_category? ? source.product.tax_category : originator.tax_category
-          return tax_category ? tax_category.tax_rates.match(source.order) : []
-        when Spree::Order
-          return originator.tax_category ? originator.tax_category.tax_rates.match(source) : []
-        end
-      else
-        find_closest_tax_rates_from_included_tax
-      end
-    end
-
-    # shipping fees and adjustments created from the admin panel have
-    # taxes set at creation in the included_tax field without relation
-    # to the corresponding TaxRate, so we look for the closest one
-    def find_closest_tax_rates_from_included_tax
-      approximation = (included_tax / (amount - included_tax))
-      return [] if approximation.infinite? or approximation.zero?
-      [Spree::TaxRate.order("ABS(amount - #{approximation})").first]
+      TaxRateFinder.new.tax_rates(originator, source, amount, included_tax)
     end
 
     def self.without_callbacks

app/services/tax_rate_finder.rb

@@ -0,0 +1,33 @@
+# Finds tax rates on which an adjustment is based on.
+# For example a packaging fee may contain VAT. This service finds the VAT rate
+# for the tax included in the packaging fee.
+class TaxRateFinder
+  # @return [Array<Spree::TaxRate>]
+  def tax_rates(originator, source, amount, included_tax)
+    case originator
+    when Spree::TaxRate
+      [originator]
+    when EnterpriseFee
+      case source
+      when Spree::LineItem
+        tax_category = originator.inherits_tax_category? ? source.product.tax_category : originator.tax_category
+        tax_category ? tax_category.tax_rates.match(source.order) : []
+      when Spree::Order
+        originator.tax_category ? originator.tax_category.tax_rates.match(source) : []
+      end
+    else
+      find_closest_tax_rates_from_included_tax(amount, included_tax)
+    end
+  end
+
+  private
+
+  # shipping fees and adjustments created from the admin panel have
+  # taxes set at creation in the included_tax field without relation
+  # to the corresponding TaxRate, so we look for the closest one
+  def find_closest_tax_rates_from_included_tax(amount, included_tax)
+    approximation = (included_tax / (amount - included_tax))
+    return [] if approximation.infinite? || approximation.zero?
+    [Spree::TaxRate.order("ABS(amount - #{approximation})").first]
+  end
+end

spec/models/spree/adjustment_spec.rb

@@ -281,21 +281,6 @@ module Spree
           adjustment.included_tax.should == 10.00
         end
       end
-
-      describe "getting the corresponding tax rate" do
-        let!(:adjustment_with_tax)    { create(:adjustment, amount: 50, included_tax: 10) }
-        let!(:adjustment_without_tax) { create(:adjustment, amount: 50, included_tax: 0) }
-        let!(:tax_rate)               { create(:tax_rate, calculator: Spree::Calculator::DefaultTax.new, amount: 0.25) }
-        let!(:other_tax_rate)         { create(:tax_rate, calculator: Spree::Calculator::DefaultTax.new, amount: 0.3) }
-
-        it "returns [] if there is no included tax" do
-          adjustment_without_tax.find_closest_tax_rates_from_included_tax.should == []
-        end
-
-        it "returns the most accurate tax rate" do
-          adjustment_with_tax.find_closest_tax_rates_from_included_tax.should == [tax_rate]
-        end
-      end
     end
 
     context "extends LocalizedNumber" do

spec/services/tax_rate_finder_spec.rb

@@ -0,0 +1,73 @@
+require 'spec_helper'
+
+describe TaxRateFinder do
+  describe "getting the corresponding tax rate" do
+    let(:amount) { BigDecimal(100) }
+    let(:included_tax) { BigDecimal(20) }
+    let(:tax_rate) { create_rate(0.2) }
+    let(:close_tax_rate) { create_rate(0.25) }
+    let(:other_tax_rate) { create_rate(0.3) }
+    let(:tax_category) { create(:tax_category, tax_rates: [tax_rate]) }
+    # This zone is used by :order_with_taxes and needs to match it
+    let(:zone) { create(:zone, name: "GlobalZone") }
+    let(:shipment) { create(:shipment) }
+    let(:enterprise_fee) { create(:enterprise_fee, tax_category: tax_category) }
+    let(:order) { create(:order_with_taxes) }
+
+    it "finds the tax rate of a shipping fee" do
+      rates = TaxRateFinder.new.tax_rates(
+        tax_rate,
+        shipment,
+        amount,
+        included_tax
+      )
+      expect(rates).to eq [tax_rate]
+    end
+
+    it "finds a close match" do
+      close_tax_rate
+      other_tax_rate
+
+      rates = TaxRateFinder.new.tax_rates(
+        nil,
+        shipment,
+        amount,
+        included_tax
+      )
+
+      expect(rates).to eq [close_tax_rate]
+    end
+
+    it "finds the tax rate of an enterprise fee" do
+      rates = TaxRateFinder.new.tax_rates(
+        enterprise_fee,
+        order,
+        amount,
+        included_tax
+      )
+      expect(rates).to eq [tax_rate]
+    end
+
+    # There is a bug that leaves orphan adjustments on an order after
+    # associated line items have been removed.
+    # https://github.com/openfoodfoundation/openfoodnetwork/issues/3127
+    pending "deals with a missing line item" do
+      rates = TaxRateFinder.new.tax_rates(
+        enterprise_fee,
+        nil,
+        amount,
+        included_tax
+      )
+      expect(rates).to eq [tax_rate]
+    end
+
+    def create_rate(amount)
+      create(
+        :tax_rate,
+        amount: amount,
+        calculator: Spree::Calculator::DefaultTax.new,
+        zone: zone
+      )
+    end
+  end
+end