'Lookup a ContentType by ID. Uses the same shared cache as get_for_model
(though ContentTypes are obviously not created on-the-fly by get_by_id).'

def get_for_id(self, id):

    try:
        ct = self.__class__._cache[self.db][id]
    except KeyError:
        ct = self.get(pk=id)
        self._add_to_cache(self.db, ct)
    return ct
