public class Customer
{
    public Customer
    {
        this.Orders = new HashSet<Orders>();
    }

    public int Id {get; set;}

    public virtual ICollection<Order> Orders{ get; set; }
}

public class Order
{
    public int Id {get; set;}

    public Customer Customer { get; set; }

    [ForeignKey("Customer ")]
    public int? CustomerId { get; set; }
}