[Zope] changing select metadata

Hello list, --- A quickie: I'm trying to change the expiration date of an object with this code: obj = getattr(context, id) obj.editMetadata(expiration_date=...) which works very well, however all other metadata gets wiped out :( Now, what's the method that works the other way, i.e. only change the provided attributes and leave the others be? \malthe