GetTotalSalesByCustomer

 public static string GetTotalSalesByCustomer(CarDealerContext context)
        {
            var custemers = context.Customers.Where(x => x.Sales.Any(y => y.Car != null))
                .OrderByDescending(x => x.Sales.Sum(y => y.Car.PartCars.Sum(s => s.Part.Price)))
                .ThenByDescending(x => x.Sales.Count())
                .Select(x => new
                {
                    fullName = x.Name,
                    boughtCars = x.Sales.Count(),
                    spentMoney = x.Sales.Sum(y=>y.Car.PartCars.Sum(s=>s.Part.Price))
                })

                .ToArray();

            var result = JsonConvert.SerializeObject(custemers, Formatting.Indented);

            return result;
        }