public function getReadIds(User $user)
    {
        return Discussion::leftJoin('discussion_user', 'discussion_user.discussion_id', '=', 'discussions.id')
            ->where('discussion_user.user_id', $user->id)
            ->whereColumn('last_read_post_number', '>=', 'last_post_number')
            ->pluck('id')
            ->all();
    }