def _ensure_object_id(cls, id):
        """Checks whether the given id is an ObjectId instance, and if not wraps it."""
        if isinstance(id, ObjectId):
            return id

        if isinstance(id, basestring) and OBJECTIDEXPR.match(id):
            return ObjectId(id)

        return id
Checks whether the given id is an ObjectId instance, and if not wraps it.