public static function getAuthors()
    {
        $query = static::find()
            ->alias('c')
            ->select(['c.createdBy', 'a.username'])
            ->joinWith('author a')
            ->groupBy(['c.createdBy', 'a.username'])
            ->orderBy('a.username')
            ->asArray()
            ->all();

        return ArrayHelper::map($query, 'createdBy', 'author.username');
    }