Multiple instances of the same model in a form -error call to a member function formName() on array

I am trying to make a form which includes several instances on the same model.
In my case the model is PollResult that is one user’s answers to a Poll (a question).
I use an indexed array of PollResults and try to use it in my form but this results in the error Call to a member function formName() on array.
The error occurs at the first field’s declaration line of the form and probably at the other fields.

Here is my controller’s code

class SurveyController extends Controller {

 public function actionAnswer($id) 
      $user_id=$username = Yii::$app->user->identity->id;
      $model=new Survey();
       echo('count is : '.$count);

      foreach ($polls as $poll) {
                 $pollResult=new PollResult();
            $pollResults['nb'.$poll->id]=$pollResult;//on utilise le poll_id pour identifier les pollResults dans le formulaire
      if (Model::loadMultiple($pollResults, Yii::$app->request->post()) && Model::validateMultiple($pollResults)) {
           foreach ($pollResults as $pollResult) {
           return $this->redirect(['answer','id'=>$id]);
      return $this->render('answer',[
           'pollResults' => $pollResults

Then answer.php

<?= $this->render('_form', ['model' => $model, 'pollResults'=>$pollResults]); ?>

Then the form itself

<?php use yii\helpers\Html; use yii\widgets\ActiveForm; use app\modules\admin\models\Poll; use app\modules\admin\models\Answer; ?>
<?php $form = ActiveForm::begin([
    'enableClientValidation' => false, 
]); ?>

      foreach($pollResults as $key => $pollResult){
           //on crée une array associative clé = id de la réponse, valeur = texte de la réponse — ceci pour les questions à choix multiples

         echo $form->field($pollResults,'[$key]user_id')->hiddenInput(['value' => Yii::$app->user->identity->id])->label(false); 

        echo $form->field($pollResults,'[$key]poll_id')->hiddenInput(['value' => $poll->id])->label(false); 
        foreach($answers as $ans){

         switch  ($poll->allow_multiple) {//0 multiple answer in list, 1 only one answer, 2 textual answer
              case 0:
                  echo $form->field($pollResults,'[$key]answer_ids')->checkboxList($items,['separator' => '<br>'])->label(false);
              case 1:
                   echo $form->field($pollResults,'[$key]answer_ids')->RadioList($items,['separator' => '<br>'])->label(false);
             case 2: 
                     echo $form->field($pollResults,'[$key]answer_ids')->textarea(['rows' => 3])->label(' Saisissez votre réponse ici');


<?= Html::submitButton('Save'); ?>
<?php ActiveForm::end(); ?>

What is the matter ? What can I do to solve this ?

I eventually made it out.
In the field method one must use the name of the model $pollResult — not the array $pollResults.
Moreover the name of the column must be written like this ‘[’.$key.’]user_id’ — not ‘[$key]user_id’
Hope this will help other people facing the same trouble
For example a full line for a field should look like this

echo $form->field($pollResult,’[’.$key.’]answer_ids’)->checkboxList($items,[‘separator’ => ‘