Entity Framework - How to save child entities

I'm using .NET 4 EF and am trying to save the child entities (InvoiceLogs) of an entity (Invoice). On form submit I have a detached invoice object that has an EntityCollection of modified InvoiceLog entities passed to my controller action. I then attach the invoice to the data context and call SaveChanges. The problem is that this only saves the Invoice, it doesn't save the modified InvoiceLog entities. If I try to attach the InvoiceLog entities separately then it gives me the following error:

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

How do I save child entities in EF 4??

//POST: /Secure/Invoices/SaveHours/
        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult SaveHours([Bind(Exclude = "InvoiceLogs")]Invoice invoice)
        {
            TryUpdateModel(invoice.InvoiceLogs, "InvoiceLogs");
            invoice.UpdateDate = DateTime.Now;
            invoice.DeveloperID = Developer.DeveloperID;
            //attaching existing invoice.
            DataContext.InvoiceData.Attach(invoice);
            //attach invoice hours.
            foreach (var log in invoice.InvoiceLogs)
            {
                DataContext.InvoiceData.AttachLog(log);
            }
            //save changes.
            DataContext.SaveChanges();
            //redirect to invoice list.
            return RedirectToAction("Index");
        }

public static void Attach(Invoice invoice)
            {
                var i = new Invoice { InvoiceID = invoice.Invoic开发者_StackOverflow中文版eID };
                db.Invoices.Attach(i);
                db.Invoices.ApplyCurrentValues(invoice);
            }

            public static void AttachLog(InvoiceLog log)
            {
                var i = new InvoiceLog { InvoiceLogID = log.InvoiceLogID };
                db.InvoiceLogs.Attach(i);
                db.InvoiceLogs.ApplyCurrentValues(log);
            }

Thanks, Justin

Got it working, I needed to populate the InvoiceLogID and InvoiceID in hidden fields so they'd get populated in the EntityCollection.