public static function getUnsyncedByAchiever($achiever)
    {
        $className = (new static)->getAchieverClassName($achiever);

        $achievements = AchievementProgress::where('achiever_type', $className)
                                           ->where('achiever_id', $achiever->id)->get();
        $synced_ids = $achievements->map(function ($el) {
            return $el->achievement_id;
        })->toArray();

        return self::whereNotIn('id', $synced_ids);
    }