public static string ImportSales(CarDealerContext context, string inputJson)
{
ICollection<Sale> salesToImport = new List<Sale>();
IEnumerable<ImportSaleDto> saleDtos = JsonConvert.DeserializeObject<ImportSaleDto[]>(inputJson);

if (saleDtos != null)
{
foreach (ImportSaleDto saleDto in saleDtos)
{
if (!IsValid(saleDto))
{
continue;
}

bool isCarIdExisting = context
.Cars
.Any(c => c.Id == saleDto.CarId);

if (!isCarIdExisting)
{
continue;
}

bool isCustomerIdExisting = context
.Customers
.Any(cu => cu.Id == saleDto.CustomerId);

if (!isCustomerIdExisting)
{
continue;
}

Sale newsale = new Sale()
{
CarId = saleDto.CarId,
CustomerId = saleDto.CustomerId,
Discount = saleDto.Discount
};

salesToImport.Add(newsale);
}

context.Sales.AddRange(salesToImport);
context.SaveChanges();
}

return $"Successfully imported {salesToImport.Count}.";
}