public function getComments($offset = 0)
    {
        // find all not equal threads for selected model
        $rows = Yii::$app->db
            ->createCommand('SELECT DISTINCT thread '.
                'FROM comment '.
                'WHERE parent_id=:parent_id AND model=:model AND status=:status '.
                'ORDER BY thread DESC '.
                'LIMIT '. Yii::$app->params['commentsPerPage'] .' OFFSET '. $offset)
            ->bindValues([
                ':parent_id' => $this->id,
                ':model' => self::COMMENT_FOR , 
                ':status' => Comment::STATUS_APPROVED,
            ])
            ->queryAll();
        // keep threads
        $a = []; foreach($rows as $row) $a[] = $row['thread'];
        // select all comments of kept threads
        return Comment::find()
            ->where('status=' . Comment::STATUS_APPROVED)
            ->andWhere(['in', 'thread', $a])
            ->orderBy('thread DESC, created_at ASC')
            ->all();
    }