fab-manager/app/workers/reservation_reminder_worker.rb

class ReservationReminderWorker
  include Sidekiq::Worker

  ## In case the reminder is enabled but no delay were configured, we use this default value
  DEFAULT_REMINDER_DELAY = 24.hours

  def perform
    enabled = Setting.find_by(name: 'reminder_enable').try(:value)
    if enabled == 'true'
      delay = Setting.find_by(name: 'reminder_delay').try(:value).try(:to_i).try(:hours) || DEFAULT_REMINDER_DELAY

      starting = DateTime.current.beginning_of_hour + delay
      ending = starting + 1.hour

      Reservation.joins(:slots).where('slots.start_at >= ? AND slots.start_at <= ? AND slots.canceled_at IS NULL', starting, ending).each do |r|
        already_sent = Notification.where(
            attached_object_type: Reservation.name,
            attached_object_id: r.id,
            notification_type_id: NotificationType.find_by_name('notify_member_reservation_reminder')
        ).count
        unless already_sent > 0
          NotificationCenter.call type: 'notify_member_reservation_reminder',
                                  receiver: r.user,
                                  attached_object: r
        end
      end
    end
  end
end