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_category(category_id):
    category = Category.query.filter_by(id=category_id).first_or_404()

    involved_users = User.query.filter(Forum.category_id == category.id,
                                       Topic.forum_id == Forum.id,
                                       Post.user_id == User.id).all()

    category.delete(involved_users)
    flash(_("Category with all associated forums deleted."), "success")
    return redirect(url_for("management.forums"))