Who should be responsible for getting the data in MVC? - asp.net-mvc-3

Question

Learning ASP.NET MVC with a new project, and a little unsure of where some things should happen. I've read that ViewModels are a Good Thing (tm) and had planned on doing it in a similar fashion anyway, but I'm still not entirely clear on the responsibilities of the Model vs. the Controller.
Should the ViewModel be responsible for actually loading itself from the ORM? Thus the controller would just call ViewModel.GetObject() and pass the result back to the view?
Or should I load the data in the Controller, and then transform it into the ViewModel? Seems like that puts a lot of work in the Controller though, which is supposed to be kept somewhat lightweight.
I guess I could also have a third party that is responsible for pulling the data, then the Controller would call that and transform it for the appropriate ViewModel.
So any thoughts on what is the "best" approach?

Answer 1

The controller will create the viewmodel object and fill it out using the model. The model should use the ORM to get the data.
The ViewModel is always specific to the view only, and the model is specific to the domain. In CQRS you would actually just get the ViewModel and send it to the view.
From the controller you can do what ever it takes to make your CRUD happen for the view. If you use the Repo pattern thats ok, if you use NHibernate or EF directly thats cool tool. Once the ViewModel goes to the view it will be disconnected from everything like the DB, so fill it out before it gets there.

Answer 2

Personally I use a repository. So the controller queries a repository and gets a model, then maps the model to a view model and passes the view model to the view. Example:
public class ProductsController: Controller
{
private readonly IProductsRepository _repository;
private readonly IMapperEngine _mapper;
public ProductsController(IProductsRepository repository, IMapperEngine mapper)
{
_repository = repository;
_mapper = mapper;
}
public ActionResult Index(int id)
{
Product product = _repository.GetProduct(id);
ProductViewModel viewModel = _mapper.Map<Product, ProductViewModel>(product);
return View(viewModel);
}
}
And because this is repetitive logic I use custom action filters:
public class ProductsController: Controller
{
private readonly IProductsRepository _repository;
public ProductsController(IProductsRepository repository)
{
_repository = repository;
}
[AutoMap(typeof(Product), typeof(ProductViewModel))]
public ActionResult Index(int id)
{
Product product = _repository.GetProduct(id);
return View(product);
}
}
in this case the custom action filter intercepts the result of the action and replaces it using the corresponding mapping layer.
The way this repository is implemented is not the responsibility of the controller (whether it is an ORM, direct SQL queries, or even distant web service calls). As long as it is injected some proper implementation it will work which allows for weaker coupling between the different parts of the application and easier unit testing in isolation. So in this example it is the implementation of the repository that is responsible for fetching data.