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

participant has met per-participant limit

Error message

participant has met per-participant limit

What it means

CalendarEvent.reserve_for raises ReservationError when the appointment group's max_appointments_per_participant is set and the participant already holds reservations equal to or exceeding that limit. It enforces the per-participant cap on an appointment group before creating a new reservation.

Solutions

  1. Cancel an existing reservation first (if it is in the future) so the count drops below the limit, then retry reserve_for.
  2. Raise max_appointments_per_participant on the appointment group via its update API if the limit is too strict.
  3. Check reservations_for(participant).size before calling and surface a friendly 'limit reached' UI message instead of attempting the booking.

Example fix

// before
slot.reserve_for(user, participant, cancel_existing: true)
// after
limit = appointment_group.max_appointments_per_participant
existing = appointment_group.reservations_for(participant)
if limit && existing.size >= limit
  raise UserFacingError, 'You have reached the booking limit for this appointment group'
end
slot.reserve_for(user, participant)
Defensive patterns

Strategy: validation

Validate before calling

limit = appointment_group.max_appointments_per_participant
if limit && appointment_group.reservations_for(participant).size >= limit
  raise UserFacingError, 'booking limit reached'
end

Try / catch

begin
  slot.reserve_for(user, participant)
rescue ReservationError => e
  if e.message.include?('per-participant limit')
    notify_limit_reached(participant)
  else
    raise
  end
end

Prevention

When it happens

Trigger: Calling reserve_for when context.max_appointments_per_participant is non-nil and context.reservations_for(participant).size >= max_appointments_per_participant.

Common situations: Student booking more appointment-group slots than the course allows; integrations or scripts looping over multiple slots; users double-submitting the booking form.

Related errors

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/bfea7ecaf6df5640. Report an issue: GitHub.

Appendix: source

Thrown at app/models/calendar_event.rb:608

  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"
        save!
      end
      context.clear_cached_available_slots!
      event
    end
  end

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