public RedirectToRouteResult AddItem(BillingViewModel mdl) { InvoiceItem i = new InvoiceItem(); int? id = db.InvoiceItem_IdMax().First(); i.id = (int)id + 1; i.invoiceId = mdl.InvoiceItem.invoiceId; i.pid = mdl.Product.pid; i.iid = mdl.Inventory.iid; i.quantity = mdl.InvoiceItem.quantity; i.free = mdl.InvoiceItem.free; db.InvoiceItems.Add(i); Inventory inv = db.Inventories.SingleOrDefault(x => x.iid == i.iid); inv.quantity -= (i.quantity + i.free); db.SaveChanges(); //return PartialView("_InvoiceAddItem", mdl); return(RedirectToAction("GenerateInvoice", new { id = mdl.InvoiceItem.invoiceId })); }