def find_by_id(cls, id):
        """
        Finds a single document by its ID. Throws a
        NotFoundException if the document does not exist (the
        assumption being if you've got an id you should be
        pretty certain the thing exists)
        """
        obj = cls.find_one(cls._id_spec(id))
        if not obj:
            raise NotFoundException(cls.collection, id)
        return obj