def fetch_existing(self, pk):
raise NotImplementedError<EOL>
Fetch an existing object that corresponds to a given ID.

        This needs to be subclassed since, depending on the object class,
        the fetching mechanism might be different. See implementations of
        SQLAEmbeddedReference and MongoEmbeddedReference for a concrete
        example of fetching objects from a relational and non-relational
        database.

        :param str pk: ID of the object that's supposed to exist.
        :returns: an instance of the object class.
        :raises: ReferenceNotFoundError if the object doesn't exist.