Related object in CreateWithRelatedMixin should be available to template on GET request
The Http404 should be raised at GET stage, and not at POST. At GET stage, we should already be reasonably sure there won't be a 404, and raise a 410 Gone at the POST stage instead.
The object should be available as
related_object in the templates after GET request, and the query should be cached so that we don't make another database hit when we need to retrieve the object later at POST stage.