public class Car
    {
        public Car()
        {
            this.Parts = new HashSet<Part>();
        }
        public int Id { get; set; }
        public string Make { get; set; }
        public string Model { get; set; }
        public Int64 TravelledDistance { get; set; }
        public virtual ICollection<Part> Parts { get; set; }
    }

    public class Customer
    {
        public Customer()
        {
            this.Sales = new HashSet<Sale>();
        }
        public int Id { get; set; }
        public string Name { get; set; }
        public DateTime BirthDate { get; set; }
        public bool IsYoungDriver { get; set; }
        public virtual ICollection<Sale> Sales { get; set; }
    }

    public class Sale
    {
        public int Id { get; set; }
        public int Discount { get; set; }
        public int CustomerId { get; set; }
        public virtual Customer Customer { get; set; }
        public int CarId { get; set; }
        public virtual Car Car { get; set; }
    }

	И методите за зареждане на данни

private static void ImportCars(CarDealerContext context)
        {
            string json = File.ReadAllText("../../Import/cars.json");
            List<Car> cars = JsonConvert.DeserializeObject<List<Car>>(json);
            Random rnd = new Random();
            List<Part> parts = context.Parts.ToList();

            foreach (var c in cars)
            {
                int partsId = rnd.Next(1, parts.Count - 1);
                Part part = context.Parts.Find(partsId);
                c.Parts.Add(part);
                partsId = rnd.Next(1, parts.Count - 1);
                c.Parts.Add(context.Parts.Find(partsId));
                partsId = rnd.Next(1, parts.Count - 1);
                c.Parts.Add(context.Parts.Find(partsId));
            }

            context.Cars.AddRange(cars);
            context.SaveChanges();
        }

 private static void ImportCustomers(CarDealerContext context)
        {
            string json = File.ReadAllText("../../Import/customers.json");
            List<Customer> customer = JsonConvert.DeserializeObject<List<Customer>>(json);
            Random rnd = new Random();
            List<Car> cars = context.Cars.ToList();

            foreach (var c in customer)
            {
                int carsId = rnd.Next(1, cars.Count - 1);
                c.Sales.Add(context.Sales.Find(carsId));
                carsId = rnd.Next(1, cars.Count - 1);
                c.Sales.Add(context.Sales.Find(carsId));
            }

            context.Customers.AddRange(customer);
            context.SaveChanges();

        }