How to map View Model back to Domain Model in a POST action?

I use an IBuilder interface and implement it using the ValueInjecter

public interface IBuilder<TEntity, TViewModel>
{
      TEntity BuildEntity(TViewModel viewModel);
      TViewModel BuildViewModel(TEntity entity);
      TViewModel RebuildViewModel(TViewModel viewModel); 
}

… (implementation) RebuildViewModel just calls BuildViewModel(BuilEntity(viewModel))

[HttpPost]
public ActionResult Update(ViewModel model)
{
   if(!ModelState.IsValid)
    {
       return View(builder.RebuildViewModel(model);
    }

   service.SaveOrUpdate(builder.BuildEntity(model));
   return RedirectToAction("Index");
}

btw I don’t write ViewModel I write Input cuz it’s much shorter, but that just not really important
hope it helps

Update:
I’m using this approach now in the ProDinner ASP.net MVC Demo App,
it’s called IMapper now, there’s also a pdf provided where this approach is explained in detail