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

participant has already reserved this appointment

Error message

participant has already reserved this appointment

What it means

CalendarEvent.reserve_for raises ReservationError when the participant already holds a reservation (child_events_for(participant) is non-empty) on this specific appointment slot. A participant can only reserve a given slot once.

Solutions

1. Treat this as success/idempotent: check child_events_for(participant) first and no-op if a reservation already exists.
2. Disable the booking button / debounce submissions in the client before re-posting.
3. If a different reservation is intended, cancel the existing one first, then call reserve_for with cancel_existing: true (only if it is in the future).

Example fix

// before
slot.reserve_for(user, participant)
// after
unless slot.child_events_for(participant).present?
  slot.reserve_for(user, participant)
end

return if slot.child_events_for(participant).present? # already reserved, treat as idempotent success

begin
  slot.reserve_for(user, participant)
rescue ReservationError => e
  raise unless e.message.include?('already reserved')
  # treat as success
end

When it happens

Trigger: Calling reserve_for twice for the same participant on the same slot; double-click on the booking button issuing two requests; a retry after a timeout when the first request actually succeeded.

Common situations: Duplicate form submissions; idempotency-missing API clients; users refreshing a confirmation page that re-posts the booking.

Related errors

• all slots filled
• cannot cancel past reservation
• ineligible participant
• invalid participant
• not an appointment

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/65d3e9eee6d85c13. Report an issue: GitHub.

Appendix: source

Thrown at app/models/calendar_event.rb:610

    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

  def child_events_for(participant)
    if child_events.loaded?

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