Proper way to validate model in ASP.NET MVC 2 and ViewModel apporach

Question

I am writing an ASP.NET MVC 2 application using NHibernate and repository pattern. I have an assembly that contains my model (business entities), moreover in my web project I want to use flattened objects (possibly with additional properties/logic) as ViewModels. These VMs contain UI-specific metadata (eg. DisplayAttribute used by Html.LabelFor() method).

The problem is that I don't know how to implement validation so that I don't repeat myself throughout various tiers (specifically validation rules are written once in Model and propagated to ViewModel).

I am using DataAnnotations on my ViewModel but this means no validation rules are imposed on the Model itself. One approach I am considering is deriving ViewModel objects from business entities adding new properties/overriding old ones, thus preserving validation metadata between the two however this is an ugly workaround.

I have seen Automapper project which helps to map properties, but I am not sure if it can handle ASP.NET MVC 2 validation metadata properly. Is it difficult to use custom validation framework in asp.net mvc 2?

Do you have any patterns that help to preserve DRY in regard to validation?

Answer 1

It is fine to repeat validation. Trick is to place it where it's appropriate.

In your case - at UI, validate UI logic (view model props must not be null, in correct format etc.), in business layer - validate business logic (account has money etc.).

Do not use DRY as an excuse to violate SRP! :P

View models are supposed to uncouple your business layer from presentation role.
Don't glue everything together again.

---

I guess Automapper can't handle that. :)