Fix: adjust tax precise amount when issuing credit notes

app/models/credit_note.rb

@@ -129,6 +129,18 @@ def sub_total_excluding_taxes_amount_cents
   end
   alias_method :sub_total_excluding_taxes_amount_currency, :currency
 
+  def precise_total
+    items.sum(&:precise_amount_cents) - precise_coupons_adjustment_amount_cents + precise_taxes_amount_cents
+  end
+
+  def taxes_rounding_adjustment
+    taxes_amount_cents - precise_taxes_amount_cents
+  end
+
+  def rounding_adjustment
+    total_amount_cents - precise_total
+  end
+
   private
 
   def ensure_number

app/models/invoice.rb

@@ -273,6 +273,8 @@ def creditable_amount_cents
       prorated_credit_amount = credit_adjustement * fee_rate
       (fee.creditable_amount_cents - prorated_credit_amount) * (fee.taxes_rate || 0)
     end.fdiv(100).round
+    # end.fdiv(100)
+    # BECAUSE OF THIS ROUND the returned value is not precise
 
     fees_total_creditable - credit_adjustement + vat
   end

app/services/credit_notes/create_service.rb

@@ -204,10 +204,24 @@ def compute_amounts_and_taxes
       credit_note.precise_coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents
       credit_note.coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents.round
       credit_note.precise_taxes_amount_cents = taxes_result.taxes_amount_cents
-      credit_note.taxes_amount_cents = taxes_result.taxes_amount_cents.round
+      adjust_credit_note_tax_rounding if credit_note_for_all_remaining_amount?
+
+      credit_note.taxes_amount_cents = credit_note.precise_taxes_amount_cents.round
       credit_note.taxes_rate = taxes_result.taxes_rate
 
       taxes_result.applied_taxes.each { |applied_tax| credit_note.applied_taxes << applied_tax }
     end
+
+    def credit_note_for_all_remaining_amount?
+      credit_note.invoice.creditable_amount_cents == 0
+    end
+
+    def adjust_credit_note_tax_rounding
+      credit_note.precise_taxes_amount_cents -= all_rounding_tax_adjustments
+    end
+
+    def all_rounding_tax_adjustments
+      credit_note.invoice.credit_notes.sum(&:taxes_rounding_adjustment)
+    end
   end
 end

app/services/credit_notes/estimate_service.rb

@@ -58,6 +58,10 @@ def validate_items
       end
     end
 
+    def valid_credit_note?
+      CreditNotes::ValidateService.new(result, item: credit_note).valid?
+    end
+
     def valid_item?(item)
       CreditNotes::ValidateItemService.new(result, item:).valid?
     end
@@ -71,24 +75,39 @@ def compute_amounts_and_taxes
       credit_note.precise_coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents
       credit_note.coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents.round
       credit_note.precise_taxes_amount_cents = taxes_result.taxes_amount_cents
-      credit_note.taxes_amount_cents = taxes_result.taxes_amount_cents.round
+      adjust_credit_note_tax_precise_rounding if credit_note_for_all_remaining_amount?
+
+      credit_note.taxes_amount_cents = credit_note.precise_taxes_amount_cents.round
       credit_note.taxes_rate = taxes_result.taxes_rate
 
       taxes_result.applied_taxes.each { |applied_tax| credit_note.applied_taxes << applied_tax }
 
       credit_note.credit_amount_cents = (
         credit_note.items.sum(&:amount_cents) -
         taxes_result.coupons_adjustment_amount_cents +
-        taxes_result.taxes_amount_cents
+        credit_note.precise_taxes_amount_cents
       ).round
 
       compute_refundable_amount
       credit_note.total_amount_cents = credit_note.credit_amount_cents
     end
 
+    def credit_note_for_all_remaining_amount?
+      credit_note.items.sum(&:precise_amount_cents) == credit_note.invoice.fees.sum(&:creditable_amount_cents)
+    end
+
+    def adjust_credit_note_tax_precise_rounding
+      credit_note.precise_taxes_amount_cents -= all_rounding_tax_adjustments
+    end
+
+    def all_rounding_tax_adjustments
+      credit_note.invoice.credit_notes.sum(&:taxes_rounding_adjustment)
+    end
+
     def compute_refundable_amount
       credit_note.refund_amount_cents = credit_note.credit_amount_cents
 
+      # invoice.refundable_amount_cents is incorrect - in our case it returns 8200, but it should be 8199...
       refundable_amount_cents = invoice.refundable_amount_cents
       return unless credit_note.credit_amount_cents > refundable_amount_cents
 

app/services/credit_notes/validate_item_service.rb

@@ -7,7 +7,9 @@ def valid?
 
       valid_item_amount?
       valid_individual_amount?
-      valid_global_amount?
+      # we don't save taxes on the credit_note item and we'll adjust them when creating credit notes, so it makes sense
+      # to check taxes on credit note level, where taxes are calculated and stored.
+      # valid_global_amount?
 
       if errors?
         result.validation_failure!(errors:)

spec/models/credit_note_spec.rb

@@ -3,7 +3,12 @@
 require 'rails_helper'
 
 RSpec.describe CreditNote, type: :model do
-  subject(:credit_note) { create(:credit_note) }
+  subject(:credit_note) do
+    create :credit_note, credit_amount_cents: 11000, total_amount_cents: 11000, taxes_amount_cents: 1000,
+      taxes_rate: 10.0, precise_taxes_amount_cents: 1000
+  end
+
+  let(:item) { create(:credit_note_item, credit_note:, precise_amount_cents: 10000, amount_cents: 1000) }
 
   it_behaves_like 'paper_trail traceable'
 
@@ -243,10 +248,35 @@
     end
   end
 
-  describe ' #sub_total_excluding_taxes_amount_cents' do
-    it 'returs the total amount without the taxes' do
-      expect(credit_note.sub_total_excluding_taxes_amount_cents)
-        .to eq(credit_note.items.sum(&:precise_amount_cents) - credit_note.precise_coupons_adjustment_amount_cents)
+  context 'when calculating depends on related items' do
+    before do
+      item
+      credit_note.reload
+    end
+
+    describe '#sub_total_excluding_taxes_amount_cents' do
+      it 'returs the total amount without the taxes' do
+        expect(credit_note.sub_total_excluding_taxes_amount_cents)
+          .to eq(credit_note.items.sum(&:precise_amount_cents) - credit_note.precise_coupons_adjustment_amount_cents)
+      end
+    end
+
+    describe '#precise_total' do
+      it 'returns the total precise amount including precise taxes' do
+        expect(credit_note.precise_total).to eq(11000)
+      end
+    end
+  end
+
+  describe '#taxes_rounding_adjustment' do
+    it 'returns the difference between taxes and precise taxes' do
+      expect(credit_note.taxes_rounding_adjustment).to eq(0)
+    end
+  end
+
+  describe '#rounding_adjustment' do
+    it 'returns the difference between credit note total and credit note precise total' do
+      expect(credit_note.taxes_rounding_adjustment).to eq(0)
     end
   end
 
@@ -328,4 +358,36 @@
       end
     end
   end
+
+  context 'when taxes are not precise' do
+    subject(:credit_note) do
+      create :credit_note, credit_amount_cents: 8200, total_amount_cents: 8200, taxes_amount_cents: 1367,
+        taxes_rate: 20.0, precise_taxes_amount_cents: 1366.6
+    end
+
+    let(:item) { create(:credit_note_item, credit_note:, precise_amount_cents: 6833, amount_cents: 6833) }
+
+    before do
+      item
+      credit_note.reload
+    end
+
+    describe '#precise_total' do
+      it 'returns the total precise amount including precise taxes' do
+        expect(credit_note.precise_total).to eq(8199.6)
+      end
+    end
+
+    describe '#taxes_rounding_adjustment' do
+      it 'returns the difference between taxes and precise taxes' do
+        expect(credit_note.taxes_rounding_adjustment).to eq(0.4)
+      end
+    end
+
+    describe '#rounding_adjustment' do
+      it 'returns the difference between credit note total and credit note precise total' do
+        expect(credit_note.taxes_rounding_adjustment).to eq(0.4)
+      end
+    end
+  end
 end