public function createUsername($username)
    {
        if (!$this->hasEnabledUsername()) {
            return null;
        }
        $usernameClass = $this->usernameClass;
        $model = $usernameClass::findOne($this->getGUID());
        if (!$model) {
            $model = $this->create($usernameClass);
            $model->setGUID($this->getGUID());
        }
        $model->content = $username;
        return $model;
    }