feat(dunning): Add deleted_at to dunning campaign threshold (#2831)

## Roadmap Task

👉 https://getlago.canny.io/feature-requests/p/set-up-payment-retry-logic

👉
https://getlago.canny.io/feature-requests/p/send-reminders-for-overdue-invoices

 ## Context

We want to automate dunning process so that our users don't have to look
at each customer to maximize their chances of being paid retrying
payments of overdue balances and sending email reminders.

We are extending dunning campaigns management to edit and delete
campaigns.

 ## Description

This change adds capabilities to soft delete dunning campaign thresholds

app/models/dunning_campaign_threshold.rb

@@ -3,12 +3,16 @@
 class DunningCampaignThreshold < ApplicationRecord
   include Currencies
   include PaperTrailTraceable
+  include Discard::Model
+  self.discard_column = :deleted_at
 
   belongs_to :dunning_campaign
 
   validates :amount_cents, numericality: {greater_than_or_equal_to: 0}
   validates :currency, inclusion: {in: currency_list}
-  validates :currency, uniqueness: {scope: :dunning_campaign_id}
+  validates :currency, uniqueness: {scope: :dunning_campaign_id}, unless: :deleted_at
+
+  default_scope -> { kept }
 end
 
 # == Schema Information
@@ -18,13 +22,15 @@ class DunningCampaignThreshold < ApplicationRecord
 #  id                  :uuid             not null, primary key
 #  amount_cents        :bigint           not null
 #  currency            :string           not null
+#  deleted_at          :datetime
 #  created_at          :datetime         not null
 #  updated_at          :datetime         not null
 #  dunning_campaign_id :uuid             not null
 #
 # Indexes
 #
-#  idx_on_dunning_campaign_id_currency_fbf233b2ae            (dunning_campaign_id,currency) UNIQUE
+#  idx_on_dunning_campaign_id_currency_fbf233b2ae            (dunning_campaign_id,currency) UNIQUE WHERE (deleted_at IS NULL)
+#  index_dunning_campaign_thresholds_on_deleted_at           (deleted_at)
 #  index_dunning_campaign_thresholds_on_dunning_campaign_id  (dunning_campaign_id)
 #
 # Foreign Keys

db/migrate/20241118165935_add_deleted_at_to_dunning_campaign_thresholds.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class AddDeletedAtToDunningCampaignThresholds < ActiveRecord::Migration[7.1]
+  def change
+    add_column :dunning_campaign_thresholds, :deleted_at, :timestamp
+
+    safety_assured do
+      add_index :dunning_campaign_thresholds, :deleted_at
+    end
+  end
+end

db/migrate/20241119110219_update_unique_index_on_dunning_campaign_thresholds.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class UpdateUniqueIndexOnDunningCampaignThresholds < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    remove_index :dunning_campaign_thresholds, %i[dunning_campaign_id currency], unique: true, algorithm: :concurrently
+
+    add_index :dunning_campaign_thresholds,
+      [:dunning_campaign_id, :currency],
+      unique: true,
+      where: "deleted_at IS NULL",
+      algorithm: :concurrently
+  end
+end

spec/models/dunning_campaign_threshold_spec.rb

@@ -12,4 +12,34 @@
   it { is_expected.to validate_numericality_of(:amount_cents).is_greater_than_or_equal_to(0) }
   it { is_expected.to validate_inclusion_of(:currency).in_array(described_class.currency_list) }
   it { is_expected.to validate_uniqueness_of(:currency).scoped_to(:dunning_campaign_id) }
+
+  describe "currency validation" do
+    let(:currency) { "EUR" }
+    let(:dunning_campaign) { create(:dunning_campaign) }
+
+    it "validates uniqueness of currency scoped to dunning_campaign_id excluding deleted records" do
+      create(:dunning_campaign_threshold, currency:, dunning_campaign:)
+      new_record = build(:dunning_campaign_threshold, currency:, dunning_campaign:)
+
+      expect(new_record).not_to be_valid
+      expect(new_record.errors[:currency]).to include("value_already_exist")
+
+      # Records with deleted_at set should not conflict
+      deleted_record = create(:dunning_campaign_threshold, :deleted, currency:, dunning_campaign:)
+      expect(deleted_record).to be_valid
+    end
+  end
+
+  describe "default scope" do
+    let(:deleted_dunning_campaign_threshold) do
+      create(:dunning_campaign_threshold, :deleted)
+    end
+
+    before { deleted_dunning_campaign_threshold }
+
+    it "only returns non-deleted dunning_campaign_threshold objects" do
+      expect(described_class.all).to eq([])
+      expect(described_class.with_discarded).to eq([deleted_dunning_campaign_threshold])
+    end
+  end
 end