Entity Framework - Saving child entities on update

I have an Invoice entity and it has child InvoiceLog entities. When I first create an Invoice and add its InvoiceLog entities and save, it works fine. However, if I then edit the Invoice and try to add additional InvoiceLog entities, it completely ignores the new InvoiceLog entities and doesn't save them at all. Any ideas what I'm doing wrong?

//POST: /Secure/Invoices/Save/
        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult Save(Invoice invoice)
        {
            invoice.UpdateDate = DateTime.Now;
            invoice.DeveloperID = Developer.DeveloperID;
            invoice.InvoiceStatusID = (int)Enums.InvoiceStatus.Open;
            if (invoice.InvoiceID == 0)
            {//inserting new invoice.
                DataContext.InvoiceData.Insert(invoice);
            }
            else
            {//attaching existing invoice.
                DataContext.InvoiceData.Attach(invoice);
            }
            AddHours(invoice);
            //save changes.
            DataContext.SaveChanges();
            //redirect to invoice list.
            return RedirectToAction("Index");
        }

        private void Add开发者_如何转开发Hours(Invoice invoice)
        {
            //get existing logs.
            IQueryable<InvoiceLog> existingLogs = null;
            if(invoice.InvoiceID > 0)
            {
                existingLogs = DataContext.InvoiceData.GetLogs(invoice.InvoiceID);
            }
            //create new logs.
            var numDays = invoice.EndDate.Subtract(invoice.StartDate).TotalDays;
            for (int k = 0; k <= numDays; k++)
            {
                //check if log already exists.
                var existingLog = existingLogs.ToList().FindIndex(l => l.LogDate == invoice.StartDate.AddDays(k));
                if (existingLog == -1)
                {
                    //add new log.
                    var log = new InvoiceLog();
                    log.CreateDate = DateTime.Now;
                    log.UpdateDate = DateTime.Now;
                    log.Hours = 0;
                    log.InvoiceID = invoice.InvoiceID;
                    log.LogDate = invoice.StartDate.AddDays(k);
                    invoice.InvoiceLogs.Add(log);
                }
            }
        }

Thanks, Justin

You can try to add InvoiceLogs items before attaching the invoice:

    else
    {//attaching existing invoice.
        AddHours(invoice);
        DataContext.InvoiceData.Attach(invoice);
    }