validate attribute of subscription

app/models/subscription.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class Subscription < ApplicationRecord
+  validates :duration, presence: true, numericality: { only_integer: true, greater_than: 0 }
+  validate :cannot_change_after_cancelled, :cannot_change_duration, on: :update
+
+  private
+
+  def cannot_change_after_cancelled
+    return if cancelled_at_was.nil?
+
+    errors.add(:cancelled_at, 'Subscription has already been cancelled')
+  end
+
+  def cannot_change_duration
+    return if changes[:duration].nil?
+
+    errors.add(:duration, 'Duration is immutable')
+  end
+end

db/migrate/20230104090306_create_subscriptions.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateSubscriptions < ActiveRecord::Migration[7.0]
+  def change
+    create_table :subscriptions do |t|
+      t.integer :duration
+      t.datetime :cancelled_at
+
+      t.timestamps
+    end
+  end
+end

test/fixtures/subscriptions.yml

@@ -0,0 +1,4 @@
+# Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+
+one:
+  duration: 1

test/models/subscription_test.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class SubscriptionTest < ActiveSupport::TestCase
+  def setup
+    @subscription = subscriptions(:one)
+  end
+
+  test 'subscription is valid' do
+    assert_predicate @subscription, :valid?
+  end
+
+  test "duration can't be changed" do
+    @subscription.duration = 8
+    assert_not_predicate @subscription, :valid?
+  end
+
+  test "duration can't be nil" do
+    subscription = Subscription.new(duration: nil)
+    assert_not_predicate subscription, :valid?
+  end
+
+  test 'duration must be integer' do
+    subscription = Subscription.new(duration: 1.1)
+    assert_not_predicate subscription, :valid?
+  end
+
+  test 'duration must be strictly positive' do
+    subscription = Subscription.new(duration: 0)
+    assert_not_predicate subscription, :valid?
+
+    subscription = Subscription.new(duration: -1)
+    assert_not_predicate subscription, :valid?
+  end
+
+  test "cancelled_at can't be changed when not nil" do
+    subscription = Subscription.new(duration: 2, cancelled_at: DateTime.now)
+    subscription.save
+
+    subscription.cancelled_at = subscription.cancelled_at + 1.day
+    assert_not_predicate subscription, :valid?
+
+    subscription.cancelled_at = nil
+    assert_not_predicate subscription, :valid?
+  end
+
+  test 'cancelled_at can be changed when nil' do
+    subscription = Subscription.new(duration: 2)
+    subscription.save
+
+    subscription.cancelled_at = DateTime.now
+    assert_predicate subscription, :valid?
+  end
+end