def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def delete_forum(forum_id):
    forum = Forum.query.filter_by(id=forum_id).first_or_404()

    involved_users = User.query.filter(Topic.forum_id == forum.id,
                                       Post.user_id == User.id).all()

    forum.delete(involved_users)

    flash(_("Forum deleted."), "success")
    return redirect(url_for("management.forums"))