Django, update the object after a prefetch_related

You can delete the entire prefetch cache on the instance:

if hasattr(publisher, '_prefetched_objects_cache'):
    del publisher._prefetched_objects_cache

If you only want to delete a particular prefetched relation:

if hasattr(publisher, '_prefetched_objects_cache'):
    publisher._prefetched_objects_cache.pop('book_set', None)