The Didl object this favorite refers to.

def reference(self):
        """The Didl object this favorite refers to."""

        # Import from_didl_string if it isn't present already. The import
        # happens here because it would cause cyclic import errors if the
        # import happened at load time.
        global _FROM_DIDL_STRING_FUNCTION  # pylint: disable=global-statement
        if not _FROM_DIDL_STRING_FUNCTION:
            from . import data_structures_entry
            _FROM_DIDL_STRING_FUNCTION = data_structures_entry.from_didl_string

        ref = _FROM_DIDL_STRING_FUNCTION(
            getattr(self, 'resource_meta_data'))[0]
        # The resMD metadata lacks a <res> tag, so we use the resources from
        # the favorite to make 'reference' playable.
        ref.resources = self.resources
        return ref