fix deletion of notification entries when deleting a user

app/models/notification.rb

@@ -5,15 +5,16 @@ class Notification < ApplicationRecord
   belongs_to :target, polymorphic: true
 
   after_create do
-    recipient.touch(:notifications_updated_at)
+    recipient.touch(:notifications_updated_at) # rubocop:disable Rails/SkipsModelValidations
   end
 
   after_update do
-    recipient.touch(:notifications_updated_at)
+    recipient.touch(:notifications_updated_at) # rubocop:disable Rails/SkipsModelValidations
   end
 
   after_destroy do
-    recipient.touch(:notifications_updated_at)
+    # recipient might not exist at this point (account deleted, records are cleaned up async)
+    recipient&.touch(:notifications_updated_at) # rubocop:disable Rails/SkipsModelValidations
   end
 
   class << self

spec/models/notification_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe Notification, type: :model do
+  describe "associations" do
+    it { should belong_to(:recipient) }
+    it { should belong_to(:target) }
+  end
+
+  describe "before_destroy" do
+    let(:user) { FactoryBot.create(:user) }
+    let(:answer) { FactoryBot.create(:answer, user: FactoryBot.create(:user)) }
+
+    it "does not fail if the user wants to delete their account" do
+      Notification::QuestionAnswered.create(recipient: user, target: answer)
+
+      # this deletes the User record and enqueues the deletion of all
+      # associated records in sidekiq
+      user.destroy!
+
+      # so let's drain the queues
+      expect { Sidekiq::Worker.drain_all }.not_to raise_error
+    end
+  end
+end