public static function suggest($keyword, $limit = 20)
    {
        $tags = self::find()
            ->where(['like', 'name', $keyword])
            ->orderBy('frequency DESC, name')
            ->limit($limit)
            ->all();
        $names = array();
        foreach ($tags as $tag) {
            $names[] = $tag->name;
        }
        return $names;
    }