Hi Helga, 

In the generalization entity object you can only validate the attributes that are available in that entity. If you want to validate attributes that are in the specialization entity you will need to have the specialization object in your context. You can achieve this by taking the generalization object and casting it to the specialization that you want to validate. Info on how to do a cast: https://docs.mendix.com/refguide/cast-object/