feat(invoice-preview): add logic for applying coupons on preview invo…

…ice (#3073)

## Context

Preview feature enables fetching first Lago invoice. This invoice is not
persisted and is calculated on the fly.

## Description

This PR adds service for applying coupons on preview invoice. The goal
was not to affect heavily main service for applying coupons on invoices.

In order not to repeat the code, some common logic is extracted s that
it can be used on both places

app/models/applied_coupon.rb

@@ -32,6 +32,13 @@ def mark_as_terminated!(timestamp = Time.zone.now)
     self.terminated_at ||= timestamp
     terminated!
   end
+
+  def remaining_amount
+    return @remaining_amount if defined?(@remaining_amount)
+
+    already_applied_amount = credits.sum(&:amount_cents)
+    @remaining_amount = amount_cents - already_applied_amount
+  end
 end
 
 # == Schema Information

app/models/fee.rb

@@ -162,6 +162,12 @@ def basic_rate_percentage?
     end
   end
 
+  def compute_precise_credit_amount_cents(credit_amount, base_amount_cents)
+    return 0 if base_amount_cents.zero?
+
+    (credit_amount * (amount_cents - precise_coupons_amount_cents)).fdiv(base_amount_cents)
+  end
+
   def sub_total_excluding_taxes_amount_cents
     amount_cents - precise_coupons_amount_cents
   end

app/services/applied_coupons/amount_service.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module AppliedCoupons
+  class AmountService < BaseService
+    def initialize(applied_coupon:, base_amount_cents:)
+      @applied_coupon = applied_coupon
+      @base_amount_cents = base_amount_cents
+
+      super
+    end
+
+    def call
+      return result.not_found_failure!(resource: 'applied_coupon') unless applied_coupon
+
+      result.amount = compute_amount
+      result
+    end
+
+    private
+
+    attr_reader :applied_coupon, :base_amount_cents
+
+    def compute_amount
+      if applied_coupon.coupon.percentage?
+        discounted_value = base_amount_cents * applied_coupon.percentage_rate.fdiv(100)
+
+        return (discounted_value >= base_amount_cents) ? base_amount_cents : discounted_value.round
+      end
+
+      if applied_coupon.recurring? || applied_coupon.forever?
+        return base_amount_cents if applied_coupon.amount_cents > base_amount_cents
+
+        applied_coupon.amount_cents
+      else
+        return base_amount_cents if applied_coupon.remaining_amount > base_amount_cents
+
+        applied_coupon.remaining_amount
+      end
+    end
+  end
+end

app/services/coupons/preview_service.rb

@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+module Coupons
+  class PreviewService < BaseService
+    def initialize(invoice:, applied_coupons:)
+      @invoice = invoice
+      @applied_coupons = applied_coupons
+
+      super
+    end
+
+    def call
+      return result.not_found_failure!(resource: 'invoice') unless invoice
+      return result.not_found_failure!(resource: 'applied_coupons') unless applied_coupons
+
+      result.credits = []
+
+      applied_coupons.each do |applied_coupon|
+        break unless invoice.sub_total_excluding_taxes_amount_cents&.positive?
+        next unless invoice.currency == applied_coupon.amount_currency
+
+        fees = fees(applied_coupon)
+
+        next if fees.none?
+
+        base_amount_cents = base_amount_cents(applied_coupon, fees)
+        credit = add_credit(applied_coupon, fees, base_amount_cents)
+
+        result.credits << credit
+        invoice.credits << credit
+      end
+
+      result.invoice = invoice
+      result
+    end
+
+    private
+
+    attr_reader :applied_coupons, :invoice
+
+    def add_credit(applied_coupon, fees, base_amount_cents)
+      credit_amount = AppliedCoupons::AmountService.call(applied_coupon:, base_amount_cents:).amount
+      new_credit = Credit.new(
+        invoice:,
+        applied_coupon:,
+        amount_cents: credit_amount,
+        amount_currency: invoice.currency,
+        before_taxes: true
+      )
+
+      fees.each do |fee|
+        unless base_amount_cents.zero?
+          fee.precise_coupons_amount_cents += fee.compute_precise_credit_amount_cents(credit_amount, base_amount_cents)
+        end
+
+        fee.precise_coupons_amount_cents = fee.amount_cents if fee.amount_cents < fee.precise_coupons_amount_cents
+      end
+
+      invoice.coupons_amount_cents += new_credit.amount_cents
+      invoice.sub_total_excluding_taxes_amount_cents -= new_credit.amount_cents
+
+      new_credit
+    end
+
+    def base_amount_cents(applied_coupon, fees)
+      if applied_coupon.coupon.limited_billable_metrics? || applied_coupon.coupon.limited_plans?
+        fees.sum(&:amount_cents)
+      else
+        invoice.sub_total_excluding_taxes_amount_cents
+      end
+    end
+
+    # TODO: update later when charges will be added to the preview
+    def fees(applied_coupon)
+      if applied_coupon.coupon.limited_billable_metrics?
+        Fee.none
+      elsif applied_coupon.coupon.limited_plans?
+        plan_related_fees(applied_coupon)
+      else
+        invoice.fees
+      end
+    end
+
+    def plan_related_fees(applied_coupon)
+      if applied_coupon.coupon.plans.map(&:id).include?(invoice.subscriptions[0].plan_id)
+        invoice.fees
+      else
+        Fee.none
+      end
+    end
+  end
+end

app/services/credits/applied_coupon_service.rb

@@ -18,7 +18,7 @@ def call
       return result if already_applied?
       return result unless fees.any?
 
-      credit_amount = compute_amount
+      credit_amount = AppliedCoupons::AmountService.call(applied_coupon:, base_amount_cents:).amount
 
       new_credit = Credit.create!(
         invoice:,
@@ -30,9 +30,7 @@ def call
 
       fees.reload.each do |fee|
         unless base_amount_cents.zero?
-          fee.precise_coupons_amount_cents += (
-            credit_amount * (fee.amount_cents - fee.precise_coupons_amount_cents)
-          ).fdiv(base_amount_cents)
+          fee.precise_coupons_amount_cents += fee.compute_precise_credit_amount_cents(credit_amount, base_amount_cents)
         end
 
         fee.precise_coupons_amount_cents = fee.amount_cents if fee.amount_cents < fee.precise_coupons_amount_cents
@@ -72,36 +70,11 @@ def already_applied?
       invoice.credits.where(applied_coupon_id: applied_coupon.id).exists?
     end
 
-    def compute_amount
-      if applied_coupon.coupon.percentage?
-        discounted_value = base_amount_cents * applied_coupon.percentage_rate.fdiv(100)
-
-        return (discounted_value >= base_amount_cents) ? base_amount_cents : discounted_value.round
-      end
-
-      if applied_coupon.recurring? || applied_coupon.forever?
-        return base_amount_cents if applied_coupon.amount_cents > base_amount_cents
-
-        applied_coupon.amount_cents
-      else
-        return base_amount_cents if remaining_amount > base_amount_cents
-
-        remaining_amount
-      end
-    end
-
-    def remaining_amount
-      return @remaining_amount if @remaining_amount
-
-      already_applied_amount = applied_coupon.credits.sum(:amount_cents)
-      @remaining_amount = applied_coupon.amount_cents - already_applied_amount
-    end
-
     def should_terminate_applied_coupon?(credit_amount)
       return false if applied_coupon.forever?
 
       if applied_coupon.once?
-        applied_coupon.coupon.percentage? || credit_amount >= remaining_amount
+        applied_coupon.coupon.percentage? || credit_amount >= applied_coupon.remaining_amount
       else
         applied_coupon.frequency_duration_remaining <= 0
       end

app/services/invoices/preview_service.rb

@@ -2,9 +2,10 @@
 
 module Invoices
   class PreviewService < BaseService
-    def initialize(customer:, subscription:)
+    def initialize(customer:, subscription:, applied_coupons: [])
       @customer = customer
       @subscription = subscription
+      @applied_coupons = applied_coupons
 
       super
     end
@@ -25,7 +26,7 @@ def call
         created_at: Time.current,
         updated_at: Time.current
       )
-
+      invoice.credits = []
       invoice.subscriptions = [subscription]
 
       add_subscription_fee
@@ -37,7 +38,7 @@ def call
 
     private
 
-    attr_accessor :customer, :subscription, :invoice
+    attr_accessor :customer, :subscription, :invoice, :applied_coupons
 
     def boundaries
       {
@@ -83,6 +84,12 @@ def add_subscription_fee
 
     def compute_tax_and_totals
       invoice.fees_amount_cents = invoice.fees.sum(&:amount_cents)
+      invoice.sub_total_excluding_taxes_amount_cents = invoice.fees_amount_cents
+
+      if invoice.fees_amount_cents&.positive? && applied_coupons.present?
+        Coupons::PreviewService.call(invoice:, applied_coupons:)
+      end
+
       invoice.sub_total_excluding_taxes_amount_cents = invoice.fees_amount_cents - invoice.coupons_amount_cents
 
       invoice.fees.each do |fee|

spec/models/applied_coupon_spec.rb

@@ -6,4 +6,15 @@
   subject(:applied_coupon) { build(:applied_coupon) }
 
   it_behaves_like 'paper_trail traceable'
+
+  describe '#remaining_amount' do
+    let(:credit) { build(:credit, applied_coupon:, amount_cents: 10) }
+    let(:applied_coupon) { build(:applied_coupon, amount_cents: 50) }
+
+    it 'returns correct amount' do
+      applied_coupon.credits = [credit]
+
+      expect(applied_coupon.remaining_amount).to eq(40)
+    end
+  end
 end

spec/models/fee_spec.rb

@@ -403,6 +403,20 @@
     end
   end
 
+  describe 'compute_precise_credit_amount_cents' do
+    subject(:fee) { create(:add_on_fee, amount_cents: 500, precise_coupons_amount_cents: 100) }
+
+    it 'returns correct value' do
+      expect(fee.compute_precise_credit_amount_cents(10, 5)).to eq(800)
+    end
+
+    context 'when base_amount_cents is zero' do
+      it 'returns zero' do
+        expect(fee.compute_precise_credit_amount_cents(10, 0)).to eq(0)
+      end
+    end
+  end
+
   describe '#creditable_amount_cents' do
     let(:fee) { create(:fee, fee_type:, amount_cents:, invoice:) }
     let(:invoice) { create(:invoice, invoice_type: :credit) }