def _update_callback(model_class, sender, instance, created, **kwargs):
create_metadata_instance(model_class, instance)<EOL>
Callback to be attached to a post_save signal, updating the relevant
        metadata, or just creating an entry. 

        NB:
        It is theoretically possible that this code will lead to two instances
        with the same generic foreign key.  If you have non-overlapping URLs,
        then this shouldn't happen.
        I've held it to be more important to avoid double path entries.