Implement post-release one login email

app/mailers/candidate_mailer.rb

@@ -522,6 +522,15 @@ def one_login_is_coming(application_form)
     )
   end
 
+  def one_login_has_arrived(application_form)
+    @application_form = application_form
+
+    email_for_candidate(
+      @application_form,
+      subject: I18n.t!('candidate_mailer.one_login_has_arrived.subject'),
+    )
+  end
+
 private
 
   def email_for_candidate(application_form, args = {})

app/views/candidate_mailer/one_login_has_arrived.text.erb

@@ -0,0 +1,19 @@
+<% if @application_form.first_name.present? %>
+  Hello <%= @application_form.first_name %>,
+<% end %>
+
+# How you sign in to Apply for teacher training has changed
+
+You need to sign in using GOV.UK One Login. You can create a GOV.UK One Login if you do not already have one.
+
+You should use the same email address to create your GOV.UK One Login that you used to sign in to Apply for teacher training before this change. This is so you keep the existing information in your account.
+
+If you use a different email address you’ll be able to transfer your account details after you sign in.
+
+# What is GOV.UK One Login?
+
+GOV.UK One Login allows you to sign in to some government services using the same email address and password.
+
+In the future you’ll be able to use your GOV.UK One Login to access all services on GOV.UK.
+
+<%= render 'unsubscribe_from_emails_like_this' %>

app/workers/send_candidate_one_login_has_arrived_email_worker.rb

@@ -0,0 +1,42 @@
+class SendCandidateOneLoginHasArrivedEmailWorker
+  include Sidekiq::Worker
+
+  def perform
+    return if should_not_perform?
+
+    BatchDelivery.new(relation:, batch_size: 200).each do |batch_time, application_forms|
+      SendOneLoginHasArrivedEmailBatchWorker.perform_at(batch_time, application_forms.pluck(:id))
+    end
+  end
+
+  def relation
+    ApplicationForm
+      .current_cycle
+      .where(candidate_id: candidate_ids)
+      .has_not_received_email('candidate_mailer', 'one_login_has_arrived')
+      .distinct
+  end
+
+private
+
+  def candidate_ids
+    Candidate
+      .for_marketing_or_nudge_emails
+      .where.missing(:one_login_auth)
+      .pluck(:id)
+  end
+
+  def should_not_perform?
+    FeatureFlag.inactive?(:one_login_candidate_sign_in) || OneLogin.bypass?
+  end
+end
+
+class SendOneLoginHasArrivedEmailBatchWorker
+  include Sidekiq::Worker
+
+  def perform(application_form_ids)
+    ApplicationForm.where(id: application_form_ids).find_each do |application_form|
+      CandidateMailer.one_login_has_arrived(application_form).deliver_later
+    end
+  end
+end

config/clock.rb

@@ -52,6 +52,7 @@ class Clock
   every(1.day, 'SendApplyToMultipleCoursesWhenInactiveEmailToCandidatesWorker', at: '10:00') { SendApplyToMultipleCoursesWhenInactiveEmailToCandidatesWorker.perform_async }
   every(1.day, 'DfE::Analytics::EntityTableCheckJob', at: '00:30') { DfE::Analytics::EntityTableCheckJob.perform_later }
   every(1.day, 'SendCandidateOneLoginIsComingEmailWorker', at: '00:31') { SendCandidateOneLoginIsComingEmailWorker.perform_async }
+  every(1.day, 'SendCandidateOneLoginHasArrivedEmailWorker', at: '00:05') { SendCandidateOneLoginHasArrivedEmailWorker.perform_async }
 
   # End of cycle application choice status jobs
   # Changes unsubmitted application choices to 'application_not_sent'

config/locales/emails/candidate_mailer.yml

@@ -97,3 +97,5 @@ en:
       subject: Increase your chances of receiving an offer for teacher training
     one_login_is_coming:
       subject: How you sign in to Apply for teacher training is changing
+    one_login_has_arrived:
+      subject: How you sign in to Apply for teacher training has changed

spec/mailers/previews/candidate_mailer_preview.rb

@@ -554,6 +554,11 @@ def one_login_is_coming
     CandidateMailer.one_login_is_coming(application_form)
   end
 
+  def one_login_has_arrived
+    application_form = FactoryBot.build(:application_form, first_name: 'Rocket the dog')
+    CandidateMailer.one_login_has_arrived(application_form)
+  end
+
 private
 
   def candidate

spec/system/support_interface/docs_spec.rb

@@ -43,6 +43,7 @@ def and_it_contains_documentation_for_all_emails
     emails_outside_of_states = %w[
       provider_mailer-fallback_sign_in_email
       candidate_mailer-one_login_is_coming
+      candidate_mailer-one_login_has_arrived
       candidate_mailer-eoc_first_deadline_reminder
       candidate_mailer-eoc_second_deadline_reminder
       candidate_mailer-application_deadline_has_passed

spec/workers/send_candidate_one_login_has_arrived_email_worker_spec.rb

@@ -0,0 +1,45 @@
+require 'rails_helper'
+
+RSpec.describe SendCandidateOneLoginHasArrivedEmailWorker do
+  before { allow(OneLogin).to receive(:bypass?).and_return(false) }
+
+  describe '#perform' do
+    context 'feature flag is inactivated' do
+      before { FeatureFlag.deactivate('one_login_candidate_sign_in') }
+
+      it 'does not enqueue the batch worker' do
+        create(:application_form)
+
+        allow(SendOneLoginHasArrivedEmailBatchWorker).to receive(:perform_at)
+        described_class.new.perform
+        expect(SendOneLoginHasArrivedEmailBatchWorker).not_to have_received(:perform_at)
+      end
+    end
+
+    context 'feature flag is activated' do
+      before { FeatureFlag.activate('one_login_candidate_sign_in') }
+
+      it 'enqueues the batch worker with expected application forms' do
+        # Last year's application
+        create(:application_form, recruitment_cycle_year: RecruitmentCycle.previous_year)
+        # Unsubscribed candidate
+        create(:application_form, candidate: build(:candidate, unsubscribed_from_emails: true))
+        # Candidate with submission blocked
+        create(:application_form, candidate: build(:candidate, submission_blocked: true))
+        # Candidate with locked account
+        create(:application_form, candidate: build(:candidate, account_locked: true))
+        # Candidate has already been sent the email
+        create(:email, application_form: build(:application_form), mailer: 'candidate_mailer', mail_template: 'one_login_has_arrived')
+        # Candidate who has a one_login_auth record
+        with_one_login_auth = create(:candidate, one_login_auth: build(:one_login_auth))
+        create(:application_form, candidate: with_one_login_auth)
+
+        should_receive = create(:application_form)
+
+        allow(SendOneLoginHasArrivedEmailBatchWorker).to receive(:perform_at)
+        described_class.new.perform
+        expect(SendOneLoginHasArrivedEmailBatchWorker).to have_received(:perform_at).with(kind_of(Time), [should_receive.id])
+      end
+    end
+  end
+end

spec/workers/send_one_login_has_arrived_email_batch_worker_spec.rb

@@ -0,0 +1,12 @@
+require 'rails_helper'
+
+RSpec.describe SendOneLoginHasArrivedEmailBatchWorker do
+  describe '#perform' do
+    it 'enqueues candidate emails' do
+      application_forms = create_list(:application_form, 2)
+
+      expect { described_class.new.perform(application_forms.pluck(:id)) }
+        .to have_enqueued_mail(CandidateMailer, :one_login_has_arrived).twice
+    end
+  end
+end