public void Save(InvoiceItem item)
{
item.GetValidationErrors().ThrowIfHasErrors();
var invoice = item.Id == 0 ? Db.CreateAndAdd <Invoice>() : Db.Set <Invoice>().Single(x => x.Id == item.Id);
invoice.WorkTaskId = item.TaskId;
invoice.Remarks = item.Remarks;
invoice.Created = item.Created;
Db.SaveChanges();
item.Id = invoice.Id;
}