Zapis dwóch modeli do bazy

            var item = await dbContext.Reservations.SingleOrDefaultAsync(x => x.Id == reservation.Id);

            if(item == null)
            {
                dbContext.Reservations.Add(reservation);
                dbContext.SaveChanges();

                foreach (var guest in guests)
                {
                    dbContext.Guests.Add(guest);
                    dbContext.SaveChanges();

                    var reservationGuest = new GuestReservation(guest.Id, reservation.Id);

                    dbContext.GuestReservations.Add(reservationGuest);
                    dbContext.SaveChanges();
                }
            }

SaveChangesAsync()