@havvg yes `Aura.Filter` is a validation and filtering library. So consider a registration form. We want to check whether a username already exists, I added the behaviour as mentioned in #413 . I have already some rules in `Aura.Filter` and it will fail accordingly. Before I save, I ask propel to validate it. It gives me failure when there is error. So I would like to pass these errors to Aura.Filter, so I will have the errors to all the keys and it will be easy to display in view. 

If you are interested you can try the minimal standalone form ( http://harikt.com/phpform/ ) and see how I am displaying the errors. That may give you the hint why I am trying to have a method giving key value.