MVC - validation - should ViewModel have access to DB

Question

I have a view model which should check that label of a new entity is unique (not in DB yet).

At the moment I've done it in the view model class:

     public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (PowerOrDuty != null)
        {
            if (PowerOrDuty.Identifier == null)
            {
                using (var db = new PowersAndDutiesContext()) 
                {
                    var existingLabels = db.PowersAndDuties.Select(pod => pod.Label);
                    if (existingLabels.Contains(PowerOrDuty.Label))
                    {
                        yield return new ValidationResult("Cannot create a new power or duty because another power or duty with this label already exists");
                    }
                }                    
            }
           ......

Please note that this is a small internal app with small DB and my time is limited, so the code is not perfect.

I feel that DB access from view models might be a bad practice. Should view model have direct DB access? Should it be able to call a repository to get the available labels? Should validation requiring DB access be done in a controller instead?

Answer 1

I think that accessing DB from VM is not wrong... AFAIK it is not breaking MVC concept (since it is a presentation layer concept). Said that, it could be better if you have the Validate method provided by a Service Layer.

But all the logic related to the content of the ViewModel, it is better kept in the VM than in the Controller. Cleaner controllers is better.

Answer 2

Should view model have direct DB access?

I think this should be avoided at all cost

Should it be able to call a repository to get the available labels ?

This is not the concern of a ViewModel. This would introduce some complexity in the testing of your ViewModel (which should almost need none) I guess it is a sign of trouble coming.

Should validation requiring DB access be done in a controller instead ?

Maybe, if by "DB" you mean "Repository". But what comes to mind is a separate custom validation class that you will be able to (un)plug, test, and reuse, in another controller for ajax validation, etc