Add email to candidates about login changes

app/mailers/candidate_mailer.rb

@@ -517,6 +517,15 @@ def apply_to_multiple_courses_after_30_working_days(application_form)
     )
   end
 
+  def one_login_is_coming(application_form)
+    @application_form = application_form
+
+    email_for_candidate(
+      @application_form,
+      subject: I18n.t!('candidate_mailer.one_login_is_coming.subject'),
+    )
+  end
+
 private
 
   def email_for_candidate(application_form, args = {})

app/views/candidate_mailer/one_login_is_coming.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 is changing
+
+During the week of 27 January, users will begin signing in using GOV.UK One Login. You'll be able to 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 use to sign in to Apply for teacher training. 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_is_coming_email_worker.rb

@@ -0,0 +1,36 @@
+class SendCandidateOneLoginIsComingEmailWorker
+  include Sidekiq::Worker
+
+  def perform
+    return if should_not_perform?
+
+    BatchDelivery.new(relation:, batch_size: 200).each do |batch_time, application_forms|
+      SendOneLoginIsComingEmailBatchWorker.perform_at(batch_time, application_forms.pluck(:id))
+    end
+  end
+
+  def relation
+    ApplicationForm
+      .current_cycle
+      .joins(:candidate)
+      .merge(Candidate.for_marketing_or_nudge_emails)
+      .has_not_received_email('candidate_mailer', 'one_login_is_coming')
+      .distinct
+  end
+
+private
+
+  def should_not_perform?
+    FeatureFlag.active?(:one_login_candidate_sign_in) || OneLogin.bypass?
+  end
+end
+
+class SendOneLoginIsComingEmailBatchWorker
+  include Sidekiq::Worker
+
+  def perform(application_form_ids)
+    ApplicationForm.where(id: application_form_ids).find_each do |application_form|
+      CandidateMailer.one_login_is_coming(application_form).deliver_later
+    end
+  end
+end

config/clock.rb

@@ -51,6 +51,7 @@ class Clock
   every(1.day, 'SendApplyToAnotherCourseWhenInactiveEmailToCandidatesWorker', at: '10:00') { SendApplyToAnotherCourseWhenInactiveEmailToCandidatesWorker.perform_async }
   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 }
 
   # End of cycle application choice status jobs
   # Changes unsubmitted application choices to 'application_not_sent'

config/locales/emails/candidate_mailer.yml

@@ -95,3 +95,5 @@ en:
       subject: Get help with your personal statement
     apply_to_course_after_inactivity:
       subject: Increase your chances of receiving an offer for teacher training
+    one_login_is_coming:
+      subject: How you sign into Apply for teacher training is changing 

spec/mailers/previews/candidate_mailer_preview.rb

@@ -549,6 +549,11 @@ def apply_to_multiple_courses_after_30_working_days
     CandidateMailer.apply_to_multiple_courses_after_30_working_days(application_form)
   end
 
+  def one_login_is_coming
+    application_form = FactoryBot.build(:application_form, first_name: 'Bob')
+    CandidateMailer.one_login_is_coming(application_form)
+  end
+
 private
 
   def candidate

spec/system/support_interface/docs_spec.rb

@@ -42,6 +42,7 @@ def and_i_see_the_provider_flow_documentation
   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-eoc_first_deadline_reminder
       candidate_mailer-eoc_second_deadline_reminder
       candidate_mailer-application_deadline_has_passed

spec/workers/send_candidate_one_login_is_coming_email_worker_spec.rb

@@ -0,0 +1,42 @@
+require 'rails_helper'
+
+RSpec.describe SendCandidateOneLoginIsComingEmailWorker do
+  before { allow(OneLogin).to receive(:bypass?).and_return(false) }
+
+  describe '#perform' do
+    context 'feature flag is activated' do
+      before { FeatureFlag.activate('one_login_candidate_sign_in') }
+
+      it 'does not enqueue the batch worker' do
+        create(:application_form)
+
+        allow(SendOneLoginIsComingEmailBatchWorker).to receive(:perform_at)
+        described_class.new.perform
+        expect(SendOneLoginIsComingEmailBatchWorker).not_to have_received(:perform_at)
+      end
+    end
+
+    context 'feature flag is deactivated' do
+      before { FeatureFlag.deactivate('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_is_coming')
+
+        should_receive = create(:application_form)
+
+        allow(SendOneLoginIsComingEmailBatchWorker).to receive(:perform_at)
+        described_class.new.perform
+        expect(SendOneLoginIsComingEmailBatchWorker).to have_received(:perform_at).with(kind_of(Time), [should_receive.id])
+      end
+    end
+  end
+end

spec/workers/send_one_login_is_coming_email_batch_worker_spec.rb

@@ -0,0 +1,12 @@
+require 'rails_helper'
+
+RSpec.describe SendOneLoginIsComingEmailBatchWorker 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_is_coming).twice
+    end
+  end
+end