controllers/RegistrationController.php
public function actionConnect($account_id)
    {
        $account = $this->finder->findAccountById($account_id);

        if ($account === null || $account->getIsConnected()) {
            throw new NotFoundHttpException;
        }

        /** @var User $user */
        $user = \Yii::createObject([
            'class'    => User::className(),
            'scenario' => 'connect'
        ]);

        if ($user->load(\Yii::$app->request->post()) && $user->create()) {
            $account->user_id = $user->id;
            $account->save(false);
            \Yii::$app->user->login($user, $this->module->rememberFor);
            return $this->goBack();
        }

        return $this->render('connect', [
            'model'   => $user,
            'account' => $account
        ]);
    }
public function actionConnect($account_id)
    {
        $account = $this->finder->findAccountById($account_id);

        if ($account === null || $account->getIsConnected()) {
            throw new NotFoundHttpException;
        }

        /** @var User $user */
        $user = \Yii::createObject([
            'class'    => User::className(),
            'scenario' => 'connect'
        ]);

        if ($user->load(\Yii::$app->request->post()) && $user->create()) {
            $account->user_id = $user->id;
            $account->save(false);
            \Yii::$app->user->login($user, $this->module->rememberFor);
            return $this->goBack();
        }

        return $this->render('connect', [
            'model'   => $user,
            'account' => $account
        ]);
    }
Displays page where user can create new account that will be connected to social account.
@param  integer $account_id
@return string
@throws NotFoundHttpException