private List<CarDTO> BuildCarsToDelete(IList<CarDTO> newCars, IList<CarDTO> existingCars)
    {
        var missingCars = new List<CarDTO>();

        var cars = newCars.Select(c => c.CarId);
        var newCarIds = new HashSet<int>(cars);

        foreach (var car in existingCars)
        {
            //If there are no new cars then it had some and they have been removed
            if (newCars.Count() == 0)
            {
                missingCars.Add(car);
            }
            else
            {
                if (!newCarIds.Contains(car.CarId))
                {
                    missingCars.Add(car);
                }
            }
        }

        return missingCars;
    }