instructure/canvas-lms · error · CalendarEvent::ReservationError

cannot cancel past reservation

Error message

cannot cancel past reservation

What it means

CalendarEvent.reserve_for raises ReservationError when options[:cancel_existing] is set and one of the participant's existing reservations ends in the past. The code intentionally refuses to destroy reservations whose end_at is before Time.now.utc, since canceling a completed appointment would falsify history.

Solutions

1. Remove options[:cancel_existing], or first delete the past reservation via the reservations API (past reservations can be deleted directly) before calling reserve_for.
2. Only pass cancel_existing: true when the participant's existing reservations are all in the future.
3. Use the appointment group's update_participants or a manual destroy of the stale reservation in the same transaction.

Example fix

// before
calendar_event.reserve_for(user, participant, cancel_existing: true)
// after
if participant.reservations.any? { |r| r.end_at < Time.now.utc }
  participant.reservations.where('end_at < ?', Time.now.utc).each(&:destroy)
end
calendar_event.reserve_for(user, participant, cancel_existing: true)

past = appointment_group.reservations_for(participant).any? { |r| r.end_at < Time.now.utc }
raise UserFacingError, 'past reservation must be removed separately' if past && options[:cancel_existing]

def cancellable?(reservation) = reservation.end_at >= Time.now.utc

begin
  slot.reserve_for(user, participant, cancel_existing: true)
rescue ReservationError => e
  handle_past_reservation_conflict(e)
end

When it happens

Trigger: Calling reserve_for on an appointment slot with options[:cancel_existing] => true while the participant holds a reservation whose end_at is earlier than the current UTC time.

Common situations: Rebooking a participant via the scheduler API after their old appointment already occurred; bulk-reschedule scripts that cancel existing reservations without filtering out past ones; UI flows letting users 'switch' into a new slot when their previous slot expired.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• participant has met per-participant limit
• all slots filled
• ineligible participant
• invalid participant
• not an appointment

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/3482bfc3330041d5. Report an issue: GitHub.

Appendix: source

Thrown at app/models/calendar_event.rb:601

  end

  def account
    (context_type == "Account") ? context : nil
  end

  class ReservationError < StandardError; end

  def reserve_for(participant, user, options = {})
    raise ReservationError, "not an appointment" unless context_type == "AppointmentGroup"
    raise ReservationError, "ineligible participant" unless context.eligible_participant?(participant)

    transaction do
      lock! # in case two people two participants try to grab the same slot
      participant.lock! # in case two people try to make a reservation for the same participant

      if options[:cancel_existing]
        context.reservations_for(participant).lock.each do |reservation|
          raise ReservationError, "cannot cancel past reservation" if reservation.end_at < Time.now.utc

          reservation.updating_user = user
          reservation.destroy
        end
      end

      raise ReservationError, "participant has met per-participant limit" if context.max_appointments_per_participant && context.reservations_for(participant).size >= context.max_appointments_per_participant
      raise ReservationError, "all slots filled" if participants_per_appointment && child_events.size >= participants_per_appointment
      raise ReservationError, "participant has already reserved this appointment" if child_events_for(participant).present?

      event = child_events.build
      event.updating_user = user
      event.context = participant
      event.workflow_state = :locked
      event.comments = options[:comments]
      event.save!
      if active?
        self.workflow_state = "locked"

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