Hi. I'm trying to introduce the Linq2SQL technique into the project and have encountered the following issue: I've created ORM classes based on the Northwind database. In some other application I create several classes derived from the Linq2SQL classes. I'm able to add such a class to EntitySet but the application fails to submit changes. Is there any way around this?

Sample code(MyClass is derived from the Order):

DataClasses1DataContext northwind = new DataClasses1DataContext();

        Product chai = northwind.Products.Single(p => p.ProductName == "Chai");
        Product tofu = northwind.Products.Single(p => p.ProductName == "Tofu");

        Order myOrder = new Order();
        myOrder.OrderDate = DateTime.Now;
        myOrder.RequiredDate = DateTime.Now.AddDays(1);
        myOrder.Freight = 34;

        Order_Detail myItem1 = new Order_Detail();
        myItem1.Product = chai;
        myItem1.Quantity = 12345;

        Order_Detail myItem2 = new Order_Detail();
        myItem2.Product = tofu;
        myItem2.Quantity = 3;

        myOrder.Order_Details.Add(myItem1);
        myOrder.Order_Details.Add(myItem2);

        Customer myCustomer = northwind.Customers.Single(c => c.CompanyName == "B's Beverages");
        MyClass newOrder = new MyClass();
        newOrder.OrderDate = DateTime.Now;
        newOrder.RequiredDate = DateTime.Now.AddDays(31);
        newOrder.Freight = 35;
        Order_Detail myItem3 = new Order_Detail();
        myItem3.Product = tofu;
        myItem3.Quantity = 3;
        newOrder.Order_Details.Add(myItem3);
        myCustomer.Orders.Add(myOrder);
        myCustomer.Orders.Add(newOrder);

As I said I'm able to add the newOrder object but unable to submit into the database.