public function loginUsername()
    {
        $loginBy    = config('taki.login_by');
        $loginField = config('taki.field.both');
        if ($loginBy === 'email') {
            $loginField = config('taki.field.email');
        } elseif ($loginBy === 'username') {
            $loginField = config('taki.field.username');
        }

        return $loginField;
    }