update calculation of item amount reported to anrok to apply coupons

app/models/credit_note_item.rb

@@ -11,6 +11,16 @@ class CreditNoteItem < ApplicationRecord
   def applied_taxes
     credit_note.applied_taxes.where(tax_code: fee.applied_taxes.select('fees_taxes.tax_code'))
   end
+
+  # This method returns item amount with coupons applied
+  # coupons are applied proportionally to the way they're applied on corresponding fee
+  # so knowing the item total proportion to fee total we can calculate item amount with coupons
+  def sub_total_excluding_taxes_amount_cents
+    return 0 if amount_cents.zero? || fee.amount_cents.zero?
+
+    item_proportion_to_fee = amount_cents.to_f / fee.amount_cents
+    item_proportion_to_fee * fee.sub_total_excluding_taxes_amount_cents
+  end
 end
 
 # == Schema Information

app/services/integrations/aggregator/taxes/credit_notes/payload.rb

@@ -51,18 +51,14 @@ def cn_item(item)
             {
               'item_id' => fee.item_id,
               'item_code' => mapped_item.external_id,
-              'amount_cents' => calculate_amount_with_coupons(item, fee)(item.amount_cents&.to_i || 0) * -1
+              'amount_cents' => item.sub_total_excluding_taxes_amount_cents * -1
             }
           end
 
           private
 
           attr_reader :customer, :integration_customer, :credit_note
 
-          def calculate_amount_with_coupons(item, fee)
-            item_proportion_to_fee = fee.amount_cents / item.amount_cents.to_f
-            item_proportion_to_fee * fee.sub_total_excluding_taxes_amount_cents
-          end
         end
       end
     end

spec/models/credit_note_item_spec.rb

@@ -1,3 +1,30 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
+
+RSpec.describe CreditNoteItem, type: :model do
+  subject(:credit_note_item) { create(:credit_note_item) }
+
+  it { is_expected.to belong_to(:credit_note) }
+  it { is_expected.to belong_to(:fee) }
+
+  describe '#sub_total_excluding_taxes_amount_cents' do
+    let(:credit_note_item) { build(:credit_note_item, amount_cents: 100, fee: fee) }
+    let(:fee) { build(:fee, amount_cents: 1000, precise_amount_cents: 1000, precise_coupons_amount_cents: 0) }
+
+    context 'when there are no coupons applied' do
+      it 'returns item amount with coupons applied' do
+        expect(credit_note_item.sub_total_excluding_taxes_amount_cents).to eq(100)
+      end
+    end
+
+    context 'when there are coupons applied' do
+      let(:fee) { build(:fee, amount_cents: 1000, precise_amount_cents: 1000, precise_coupons_amount_cents: 20) }
+
+      it 'returns item amount with coupons applied' do
+        expect(credit_note_item.sub_total_excluding_taxes_amount_cents).to eq(98)
+      end
+    end
+  end
+end
+

spec/services/integrations/aggregator/taxes/credit_notes/payload_spec.rb

@@ -45,7 +45,8 @@
       invoice:,
       add_on:,
       created_at: current_time - 3.seconds,
-      amount_cents: 200
+      amount_cents: 200,
+      precise_amount_cents: 200
     )
   end
   let(:fee_add_on_two) do
@@ -54,7 +55,9 @@
       invoice:,
       add_on: add_on_two,
       created_at: current_time - 2.seconds,
-      amount_cents: 200
+      amount_cents: 200,
+      precise_amount_cents: 200,
+      precise_coupons_amount_cents: 20
     )
   end
   let(:credit_note) do
@@ -99,7 +102,7 @@
           {
             'item_id' => fee_add_on_two.item_id,
             'item_code' => '1',
-            'amount_cents' => -180
+            'amount_cents' => -162
           }
         ]
       }