public SingleVisit SaveSingleVisit(string licencePlate)
        {
            SingleVisit visit   = SingleVisits.SingleOrDefault(x => x.ExitDate == null && x.Vehicle.NumberPlate == licencePlate);
            Vehicle     vehicle = Vehicles.SingleOrDefault(x => x.NumberPlate == licencePlate);

            if (vehicle == null) //nie ma abo i pierwszy raz wjeżdża
            {
                vehicle = new Vehicle {
                    NumberPlate = licencePlate
                };
                Vehicles.Add(vehicle);
            }
            if (visit == null) //entry
            {
                visit = new SingleVisit
                {
                    EntryDate = DateTime.Now,
                    Vehicle   = vehicle
                };
                SingleVisits.Add(visit);
            }
            else //exit
            {
                visit.ExitDate = DateTime.Now;
                if (visit.Vehicle.Owner == null) //nie ma abonamentu
                {
                    TimeSpan visitTime = (DateTime)visit.ExitDate - visit.EntryDate;
                    visit.Price             = CalculationHelpers.CauntThePrice(visitTime, Prices.ToList());
                    visit.Vehicle.TotalPay += (double)visit.Price;
                }
            }
            SaveChanges();
            return(visit);
        }