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I am using ASP.NET MVC 3 and I am using FluentValidation to validate my view models. I am just a little concerned that I might not be on the correct track. As far as what I know, model validation should be done on the domain object. Now with MVC you might have multiple view models that are similar that needs validation. What happens if a property from a domain object occurs in more than one view model? Now you are validating the same property twice, and they might not even be in sync. So if I have a User domain object then I would like to do validation on this object. Now what happens if I have UserAViewModel and UserBViewModel, so now it is multiple validations that needs to be done.

In my News class I have a property called Title, which is a required field. On my view model I also have a Title property, I use AutoMapper to map the News and NewsViewModel. So this validation is happening twice. When does domain model validation occur and when does view model validation occur?

The scenario above is just an example, so please don't critise on it.

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It's a subtle distinction but the validation on your view model is to validate correct user input and forms an anti-corruption layer for your domain model, whereas the "validation" on your domain model enforces business rules. It is perfectly normal and you should have validation on both layers. In fact it may be feasible that UserAViewModel has slightly different input validation from UserBViewModel. As for your question, generally I try to avoid exposing domain objects through my ViewModel and instead map between them (often using something like AutoMapper), that way your ViewModels truly are anticorruption layers rather than property bags of domain models. Hope that helps.

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@Paul: Yes I do map the the 2 with AutoMapper. But lets say that Title is a required input field, then do I have to mark it as required on the view model and on the domain model? When the view model is mapped to the domain object, does validation need to be done again on the domain model?
It's Raoul ;) Yes, in that scenario I would have a NotEmpty rule in the Validator for the view model and also a NullOrEmpty check in the property setter on the domain model. It may seem a little tedious, but if you later chuck out your UI or have a different interface to your domain model (e.g. web service) then you can rest assured that all your business rules are properly enforced at the domain layer.
Thanks, that was what I was thinking about, what happens if I need to share my business objects (for example).
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What happens if a property from a domain object occurs in more than one view model?

This shouldn't happen. View models should be totally divorced from your domain.

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Nope still not making sense to me. Let me give you an example. In my News class I have a property called Title, which is a required field. On my view model I also have a Title property, I use AutoMapper to map the News and NewsViewModel. So this validation is happening twice. When does domain model validation occur and when does view model validation occur. Please use my News and NewsViewModel and update your answer :)

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