Remove tax rate guessing from TaxRateFinder

🎉

app/services/tax_rate_finder.rb

@@ -8,16 +8,13 @@ class TaxRateFinder
   def self.tax_rates_of(adjustment)
     new.tax_rates(
       adjustment.originator,
-      adjustment.adjustable,
-      adjustment.amount,
-      adjustment.included_tax
+      adjustment.adjustable
     )
   end
 
   # @return [Array<Spree::TaxRate>]
-  def tax_rates(originator, adjustable, amount, included_tax)
-    find_associated_tax_rate(originator, adjustable) ||
-      find_closest_tax_rates_from_included_tax(amount, included_tax)
+  def tax_rates(originator, adjustable)
+    find_associated_tax_rate(originator, adjustable)
   end
 
   private
@@ -48,37 +45,4 @@ class TaxRateFinder
       enterprise_fee.tax_category
     end
   end
-
-  # There are two cases in which a line item is not associated to a tax rate.
-  #
-  # 1. 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.
-  # 2. Removing line items from an order doesn't always remove the associated
-  #    enterprise fees. These orphaned fees don't have a line item any more to
-  #    find the item's tax rate.
-  #
-  # In these cases we try to find the used tax rate based on the included tax.
-  # For example, if the included tax is 10% of the adjustment, we look for a tax
-  # rate of 10%. Due to rounding errors, the included tax may be 9.9% of the
-  # adjustment. That's why we call it an approximation of the tax rate and look
-  # for the closest and hopefully find the 10% tax rate.
-  #
-  # This attempt can fail.
-  #
-  # - If an admin created an adjustment with a miscalculated included tax then
-  #   we don't know which tax rate the admin intended to use.
-  # - An admin may also enter included tax that doesn't correspond to any tax
-  #   rate in the system. They may enter a fee of $1.2 with tax of $0.2, but
-  #   that doesn't mean that there is a 20% tax rate in the database.
-  # - The used tax rate may also have been deleted. Maybe the tax law changed.
-  #
-  # In either of these cases, we will find a tax rate that doesn't correspond
-  # to the included tax.
-  def find_closest_tax_rates_from_included_tax(amount, included_tax)
-    approximation = (included_tax / (amount - included_tax))
-    return [] if approximation.infinite? || approximation.zero? || approximation.nan?
-
-    [Spree::TaxRate.order(Arel.sql("ABS(amount - #{approximation})")).first]
-  end
 end

spec/services/tax_rate_finder_spec.rb

@@ -5,61 +5,33 @@ require 'spec_helper'
 describe TaxRateFinder do
   describe "getting the corresponding tax rate" do
     let(:amount) { BigDecimal(120) }
-    let(:included_tax) { BigDecimal(20) }
     let(:tax_rate) { create_rate(0.2) }
     let(:tax_category) { create(:tax_category, tax_rates: [tax_rate]) }
     let(:zone) { create(:zone_with_member) }
     let(:shipment) { create(:shipment) }
+    let(:line_item) { create(:line_item) }
     let(:enterprise_fee) { create(:enterprise_fee, tax_category: tax_category) }
     let(:order) { create(:order_with_taxes, zone: zone) }
 
     it "finds the tax rate of a shipping fee" do
-      rates = TaxRateFinder.new.tax_rates(
-        tax_rate,
-        shipment,
-        amount,
-        included_tax
-      )
+      rates = TaxRateFinder.new.tax_rates(tax_rate, shipment)
       expect(rates).to eq [tax_rate]
     end
 
-    it "finds a close match" do
+    it "deals with soft-deleted tax rates" do
       tax_rate.destroy
-      close_tax_rate = create_rate(tax_rate.amount + 0.05)
-      # other tax rates, not as close to the real one
-      create_rate(tax_rate.amount + 0.06)
-      create_rate(tax_rate.amount - 0.06)
-
-      rates = TaxRateFinder.new.tax_rates(
-        nil,
-        shipment,
-        amount,
-        included_tax
-      )
-
-      expect(rates).to eq [close_tax_rate]
+      rates = TaxRateFinder.new.tax_rates(tax_rate, shipment)
+      expect(rates).to eq [tax_rate]
     end
 
     it "finds the tax rate of an enterprise fee" do
-      rates = TaxRateFinder.new.tax_rates(
-        enterprise_fee,
-        order,
-        amount,
-        included_tax
-      )
+      rates = TaxRateFinder.new.tax_rates(enterprise_fee, order)
       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
-    it "deals with a missing line item" do
-      rates = TaxRateFinder.new.tax_rates(
-        enterprise_fee,
-        nil,
-        amount,
-        included_tax
-      )
+    it "deals with a soft-deleted line item" do
+      line_item.destroy
+      rates = TaxRateFinder.new.tax_rates(enterprise_fee, line_item)
       expect(rates).to eq [tax_rate]
     end