how to delete NDB entity using ID?

based on this docs https://developers.google.com/appengine/docs/python/ndb/entities#deleting_entities well, im still not sure why i cannot do the delete on NDB:

def get(self):
  id = self.request.get("delete")
  ndb.Key('category', id).delete()

yeah i know how to select using id

ndb.Key('category', id).get()

but its still not working...

key = ndb.Key('category', id).get()
key.delete()

this one also not working:

category.key.delete()

something wrong?