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}.";
 }