ASP.NET MVC FluentValidation with ViewModels and Business Logic Validation

Question

I'm exploring using FluentValidation as it seems to be an elegant API for validation of my ViewModels upon model binding. I'm looking for opinions on how to properly centralize validation using this library as well as from my business (service) layer and raise it up to the view without having 2 different approaches to adding modelstate errors.

I'm open to using an entirely different API but essentially looking to solve this branching validation strategy.

[Side Note: One thing I tried was to move my business method into my FluentValidation's custom RsvpViewModelValidator class and using the .Must method but it seemed wrong to hide that call in there because if I needed to actually use my Customer object they I would have to re-query it again since its out of scope]

Sample Code:

[HttpPost]
public ActionResult AcceptInvitation(RsvpViewModel model)
{
    //FluentValidation has happened on my RsvpViewModel already to check that 
    //RsvpCode is not null or whitespace
    if(ModelState.IsValid)
    {
        //now I want to see if that code matches a customer in my database.
        //returns null if not, Customer object if existing
        customer = _customerService.GetByRsvpCode(model.RsvpCode);
        if(customer == null)
        {
            //is there a better approach to this?  I don't like that I'm
            //splitting up the validation but struggling up to come up with a 
            //better way.
            ModelState.AddModelError("RsvpCode", 
                string.Format("No customer was found for rsvp code {0}", 
                              model.RsvpCode);

            return View(model);
        }

        return this.RedirectToAction(c => c.CustomerDetail());
    }

    //FluentValidation failed so should just display message about RsvpCode 
    //being required
    return View(model);
}

[HttpGet]
public ActionResult CustomerDetail()
{
     //do work.  implementation not important for this question.
}

Answer 1

To give some closure to the question (and make it acceptable) as well as summarize the comments:

Business/process logic and validation logic are two entities. Unless the validation ties in to the database (e.g. check for unique entries) there's no reason to group validation into one location. Some are responsible in the model making sure there's nothing invalid about the information, and some handle how the validated values are used within the system. Think of it in terms of property getters/setters vs the logic used in the methods with those properties.

That being said, separating out the processes (checks, error handling, etc.--anything not relating to UI) can be done in a service layer which also tends to keep the application DRY. Then the action(s) is/are only responsible for calling and presenting and not performing the actual unit of work. (also, if various actions in your application use similar logic, the checks are all in one location instead of throw together between actions. (did I remember to check that there's an entry in the customer table?))

Also, by breaking it down in to layers, you're keeping concerns modular and testable. (Accepting an RSVP isn't dependent on an action in the UI, but now it's a method in the service, which could be called by this UI or maybe a mobile application as well).

As far as bubbling errors up, I usually have a base exception that transverses each layer then I can extend it depending on purpose. You could just as easily use Enums, Booleans, out parameters, or simply a Boolean (the Rsvp either was or wasn't accepted). It just depends on how finite a response the user needs to correct the problem, or maybe change the work-flow so the error isn't a problem or something that the user need correct.

Answer 2

You can have the whole validation logic in fluent validation:

public class RsvpViewValidator : AbstractValidator<RsvpViewModel>
{
    private readonly ICustomerService _customerService = new CustomerService();
    public RsvpViewValidator()
    {
        RuleFor(x => x.RsvpCode)
            .NotEmpty()
            .Must(BeAssociatedWithCustomer)
            .WithMessage("No customer was found for rsvp code {0}", x => x.RsvpCode)
    }

    private bool BeAssociatedWithCustomer(string rsvpCode)
    {
        var customer = _customerService.GetByRsvpCode(rsvpCode);
        return (customer == null) ? false : true;
    }
}