WPF LINQ TO SQL เพิ่มข้อมูลแบบ Master - Detail ยังไงหรอคับ

                using (var ts = new TransactionScope())
                {
                    Database.Invoice creInvoice = new Database.Invoice();
                    creInvoice.invID = creID.InvoiceID(db);
                    creInvoice.cusID = tempID;
                    creInvoice.ReceiveDate = DateTime.Now;
                    creInvoice.TotalPrice = int.Parse(txbTotalPrices.Text);
                    creInvoice.TotalAmount = int.Parse(txbTotalItems.Text);
                    Database.InvoiceDetail od;
                    if (InvoiceCollection.Count() > 0)
                    {
                        foreach (var inv in InvoiceCollection)
                        {
                            od = new Database.InvoiceDetail();
                            od.invID = creInvoice.invID;
                            od.cosID = inv.CostID;
                            od.ReceiveAmount = int.Parse(inv.Amount);
                            od.Sum = int.Parse(inv.Sum);
                            creInvoice.InvoiceDetail.Add(od);// ไม่ มี method Add()ให้ใช้ เหมือน Winform  WHYYYYYYY!!!!!
                        }
                        db.Invoices.InsertOnSubmit(creInvoice);
                    }
                    db.SubmitChanges();
                    db.Connection.Close();
                    ts.Complete();
                }