Preventing the Edit of an object

Hi all.

I am a bit stuck at the moment for which may have a very simple answer. But for me I can't figuer out how to do this.

I know there is a default error handeling in entity associations. In ONE to MANY associations, we can set an on DELETE error message so that the user cant delete one object if data exists in the other.

I was wondering how to get the same error message if some one tries to EDIT an Object. In my application I have one entity CATEGORIES and the other entity ITEMS. One Category is associated with many Items.

I do not want a user to EDIT a Category when Items of that Category exists..

How to do this?

Please help

Dylan