from django.db import transaction
from django.db.models import F
from .models import Reservation, Workshop

class SoldOut(ValueError):
    pass

def reserve(*, workshop_id, user, seats):
    if not isinstance(seats, int) or isinstance(seats, bool) or not 1 <= seats <= 4:
        raise ValueError("Seats must be an integer from one to four")
    with transaction.atomic():
        changed = Workshop.objects.filter(pk=workshop_id, published=True,
            remaining__gte=seats).update(remaining=F("remaining") - seats)
        if changed != 1:
            raise SoldOut("Not enough seats")
        return Reservation.objects.create(workshop_id=workshop_id, user=user, seats=seats)

def cancel(*, reservation_id, user):
    with transaction.atomic():
        record = Reservation.objects.filter(pk=reservation_id, user=user).first()
        if record is None:
            return False
        count, _ = Reservation.objects.filter(pk=record.pk, user=user).delete()
        if count:
            Workshop.objects.filter(pk=record.workshop_id).update(remaining=F("remaining") + record.seats)
        return bool(count)