public function actionIndex()
    {
        /* @var AssignmentSearch */
        $searchModel = Yii::createObject($this->searchClass);

        if ($searchModel instanceof AssignmentSearch) {
            $dataProvider = $searchModel->search(Yii::$app->request->queryParams, $this->userIdentityClass, $this->idField, $this->usernameField);
        } else {
            $dataProvider = $searchModel->search(Yii::$app->request->queryParams);
        }

        return $this->render('index', [
            'dataProvider' => $dataProvider,
            'searchModel' => $searchModel,
            'gridViewColumns' => $this->gridViewColumns,
        ]);
    }