Move EE specific preview to its own module

app/mailers/previews/notify_preview.rb

 class NotifyPreview < ActionMailer::Preview
+  prepend EE::NotifyPreview
+
   def note_merge_request_email_for_individual_note
     note_email(:note_merge_request_email) do
       note = <<-MD.strip_heredoc
@@ -123,52 +125,6 @@ class NotifyPreview < ActionMailer::Preview
     Notify.pipeline_failed_email(pipeline, pipeline.user.try(:email))
   end
 
-  # EE-specific start
-  def add_merge_request_approver_email
-    Notify.add_merge_request_approver_email(user.id, merge_request.id, user.id).message
-  end
-
-  def issues_csv_email
-    Notify.issues_csv_email(user, project, '1997,Ford,E350', { truncated: false, rows_expected: 3, rows_written: 3 }).message
-  end
-
-  def approved_merge_request_email
-    Notify.approved_merge_request_email(user.id, merge_request.id, approver.id).message
-  end
-
-  def unapproved_merge_request_email
-    Notify.unapproved_merge_request_email(user.id, merge_request.id, approver.id).message
-  end
-
-  def mirror_was_hard_failed_email
-    Notify.mirror_was_hard_failed_email(project.id, user.id).message
-  end
-
-  def project_mirror_user_changed_email
-    Notify.project_mirror_user_changed_email(user.id, 'deleted_user_name', project.id).message
-  end
-
-  def send_admin_notification
-    Notify.send_admin_notification(user.id, 'Email subject from admin', 'Email body from admin').message
-  end
-
-  def send_unsubscribed_notification
-    Notify.send_unsubscribed_notification(user.id).message
-  end
-
-  def service_desk_new_note_email
-    cleanup do
-      note = create_note(noteable_type: 'Issue', noteable_id: issue.id, note: 'Issue note content')
-
-      Notify.service_desk_new_note_email(issue.id, note.id).message
-    end
-  end
-
-  def service_desk_thank_you_email
-    Notify.service_desk_thank_you_email(issue.id).message
-  end
-  # EE-specific end
-
   private
 
   def project
@@ -213,10 +169,4 @@ class NotifyPreview < ActionMailer::Preview
 
     email
   end
-
-  # EE-specific start
-  def approver
-    @user ||= User.first
-  end
-  # EE-specific end
 end

ee/app/mailers/preview/notify_preview.rb

+module EE
+  module NotifyPreview
+    extend ActiveSupport::Concern
+
+    # We need to define the methods on the prepender directly because:
+    # https://github.com/rails/rails/blob/3faf7485623da55215d6d7f3dcb2eed92c59c699/actionmailer/lib/action_mailer/preview.rb#L73
+    prepended do
+      def add_merge_request_approver_email
+        Notify.add_merge_request_approver_email(user.id, merge_request.id, user.id).message
+      end
+
+      def issues_csv_email
+        Notify.issues_csv_email(user, project, '1997,Ford,E350', { truncated: false, rows_expected: 3, rows_written: 3 }).message
+      end
+
+      def approved_merge_request_email
+        Notify.approved_merge_request_email(user.id, merge_request.id, approver.id).message
+      end
+
+      def unapproved_merge_request_email
+        Notify.unapproved_merge_request_email(user.id, merge_request.id, approver.id).message
+      end
+
+      def mirror_was_hard_failed_email
+        Notify.mirror_was_hard_failed_email(project.id, user.id).message
+      end
+
+      def project_mirror_user_changed_email
+        Notify.project_mirror_user_changed_email(user.id, 'deleted_user_name', project.id).message
+      end
+
+      def send_admin_notification
+        Notify.send_admin_notification(user.id, 'Email subject from admin', 'Email body from admin').message
+      end
+
+      def send_unsubscribed_notification
+        Notify.send_unsubscribed_notification(user.id).message
+      end
+
+      def service_desk_new_note_email
+        cleanup do
+          note = create_note(noteable_type: 'Issue', noteable_id: issue.id, note: 'Issue note content')
+
+          Notify.service_desk_new_note_email(issue.id, note.id).message
+        end
+      end
+
+      def service_desk_thank_you_email
+        Notify.service_desk_thank_you_email(issue.id).message
+      end
+    end
+
+    private
+
+    def approver
+      @user ||= User.first
+    end
+  end
+end