public function actionIndex()
    {
        $rates = Currency::find()->orderBy('id DESC')->all();
        $model = new Currency();

        if (Yii::$app->request->isPost) {
            if ($model->load(Yii::$app->request->post())) {

                if ($model->validate()) {
                    $model->save();
                    $this->trigger(self::EVENT_AFTER_CHANGE);
                }
                return $this->redirect(\Yii::$app->request->referrer);
            }
            else {
                throw new Exception();
            }
        }
        return $this->render('index', [
            'rates' => $rates,
            'model' => $model
        ]);
    }