Where should ASP.NET MVC 2 validation go: in the model or viewmodel classes?

Question

I am using automapper to map my models to viewmodel classes to pass to my view.

My question really is where should the validation go? I was planning on using the MetaData decorations - a feature of mvc 2.

But either in the model or viewmodel? Or in both places?

Answer 1

Validation should be done minimum at the View Model because this is what you receive as action argument and contains the user input. You could also have validation at the model.

Answer 2

My answer would be ViewModel because Model can change (for example from using Linq2SQL to EF). This way when you plug another Model you still have your validation intact.