If the RDF datastream refers to the object by the default dummy
        uriref then we need to replace that dummy reference with a real one
        before we ingest the object.

def _prepare_ingest(self):
        """If the RDF datastream refers to the object by the default dummy
        uriref then we need to replace that dummy reference with a real one
        before we ingest the object."""

        # see also commentary on DigitalObject.DUMMY_URIREF
        self.replace_uri(self.obj.DUMMY_URIREF, self.obj.uriref)