discourse/app/jobs/scheduled/pending_users_reminder.rb

require_dependency 'admin_user_index_query'

module Jobs

  class PendingUsersReminder < Jobs::Scheduled
    every 1.hour

    def execute(args)
      if SiteSetting.must_approve_users && SiteSetting.pending_users_reminder_delay >= 0
        query = AdminUserIndexQuery.new({query: 'pending', stats: false}).find_users_query # default order is: users.created_at DESC
        if SiteSetting.pending_users_reminder_delay > 0
          query = query.where('users.created_at < ?', SiteSetting.pending_users_reminder_delay.hours.ago)
        end

        newest_username = query.limit(1).pluck(:username).first

        return true if newest_username == previous_newest_username # already notified

        count = query.count

        if count > 0
          target_usernames = Group[:moderators].users.map do |u|
            u.id > 0 && u.notifications.joins(:topic)
                                       .where("notifications.id > ?", u.seen_notification_id)
                                       .where("notifications.read = false")
                                       .where("topics.subtype = '#{TopicSubtype.pending_users_reminder}'")
                                       .count == 0 ? u.username : nil
          end.compact

          unless target_usernames.empty?
            PostCreator.create(
              Discourse.system_user,
              target_usernames: target_usernames,
              archetype: Archetype.private_message,
              subtype: TopicSubtype.pending_users_reminder,
              title: I18n.t("system_messages.pending_users_reminder.subject_template", {count: count}),
              raw: I18n.t("system_messages.pending_users_reminder.text_body_template", {count: count, base_url: Discourse.base_url})
            )

            self.previous_newest_username = newest_username
          end
        end
      end

      true
    end

    def previous_newest_username
      $redis.get previous_newest_username_cache_key
    end

    def previous_newest_username=(username)
      $redis.setex previous_newest_username_cache_key, 7.days, username
    end

    def previous_newest_username_cache_key
      "pending-users-reminder:newest-username".freeze
    end

  end

end