Yii2: multiple selection of checkbox list is not saved

Question

I'm having trouble saving options for a checkbox list. When I select more than one option I have the error Answer is invalid and when I select an option it allows me to send the data but I get the error htmlspecialchars() expects parameter 1 to be string, array given

'Answer' => [
    [
        'answer' => '37',
        'question_id' => '34',
    ],
    [
        'answer' => '40',
        'question_id' => '35',
    ],
    [
        'answer' => [ //selected options from checkboxlist
            '43',
            '46',
        ],
        'question_id' => '36',
    ],
    [
        'question_id' => '37',
        'answer' => '42',
    ],
],

Controller

public function actionQuestionnaire($id)
{
    ...... 
    ........

    $count = count(Yii::$app->request->post('Answer', []));

    $answers = [new Answer()];

    for($i = 1; $i < $count; $i++) {
        $answers[] = new Answer();
    }

    if (Model::loadMultiple($answers, Yii::$app->request->post()) && Model::validateMultiple($answers)) {

        foreach ($answers as $answer) {
            $answer->save(false);
        }
        return $this->redirect('index');
    }
    return $this->render('create', [
        'model' => $model,//question data
        'answer' => $answer,
        'dataProvider'=> $dataProvider,
    ]);
}

_form

<?php $form = ActiveForm::begin([
'enableClientValidation' => false,
'enableAjaxValidation' => true,]) ?>

<?= ListView::widget([
    'layout' => '<div class="pull-left">{items}</div>',
    'dataProvider' => $dataProvider,
    'itemView' => function ($model, $key, $index, $widget) use ($form,$answer) {
        return $this->render('_answers',[
            'model' => $model, 
            'answer' => $answer,
            'index' => $index
        ]);
    },
]); ?><div class="form-group">
<?php echo Html::submitButton('<span class="fa fa-plus"></span>'.' '.Yii::t('backend', 'Send') , ['class' => 'btn btn-primary']) ?>

_form_quest

<?php
     $idq = $model->id_question;
     $count = $model->getAnswer($idq);
     echo \yii\helpers\Html::activeHiddenInput($answer, "[$index]question_id",['value' => $model->id_question]); 
     echo  $count > 1?$form->field($answer, "[$index]answer")->checkboxList(ArrayHelper::map($model->questOptions, 'id_option', 'title_option'))->label('') : $form->field($answer, "[$index]answer")->radioList(ArrayHelper::map($model->questOptions, 'id_option', 'title_option'))->label('') ;
                                    ?>  

In order to solve the problem I tried to add the rule ['answer', 'each', 'rule' => ['string']] in the model but it won't let me send the data.

Rules in model

return [
        [['question_id'], 'required'],
        [['pregunta_id'], 'integer'],
        //['answer', 'each', 'rule' => ['string']], //if you used this rule it throws me an error "Answer is invalid" when selecting an option
        [['answer'], 'string'],
    ];

When selecting more than two options

When selecting an option it allows me to send but it shows me the following error

If I only save options from the radioboxlist it allows me to save without problems, the only drawback is with the checkboxlist. I hope you can guide me with this problem.

var_dump to objet

array(4) {

[0]=>
  object(common\models\Answer)#221 (10) {
    ["_attributes":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_oldAttributes":"yii\db\BaseActiveRecord":private]=>
    NULL
    ["_related":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_relationsDependencies":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_errors":"yii\base\Model":private]=>
    NULL
    ["_validators":"yii\base\Model":private]=>
    NULL
    ["_scenario":"yii\base\Model":private]=>
    string(7) "default"
    ["_events":"yii\base\Component":private]=>
    array(0) {
    }
    ["_eventWildcards":"yii\base\Component":private]=>
    array(0) {
    }
    ["_behaviors":"yii\base\Component":private]=>
    array(0) {
    }
  }
  [1]=>
  object(common\models\Answer)#222 (10) {
    ["_attributes":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_oldAttributes":"yii\db\BaseActiveRecord":private]=>
    NULL
    ["_related":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_relationsDependencies":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_errors":"yii\base\Model":private]=>
    NULL
    ["_validators":"yii\base\Model":private]=>
    NULL
    ["_scenario":"yii\base\Model":private]=>
    string(7) "default"
    ["_events":"yii\base\Component":private]=>
    array(0) {
    }
    ["_eventWildcards":"yii\base\Component":private]=>
    array(0) {
    }
    ["_behaviors":"yii\base\Component":private]=>
    array(0) {
    }
  }
  [2]=>
  object(common\models\Answer)#223 (10) {
    ["_attributes":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_oldAttributes":"yii\db\BaseActiveRecord":private]=>
    NULL
    ["_related":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_relationsDependencies":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_errors":"yii\base\Model":private]=>
    NULL
    ["_validators":"yii\base\Model":private]=>
    NULL
    ["_scenario":"yii\base\Model":private]=>
    string(7) "default"
    ["_events":"yii\base\Component":private]=>
    array(0) {
    }
    ["_eventWildcards":"yii\base\Component":private]=>
    array(0) {
    }
    ["_behaviors":"yii\base\Component":private]=>
    array(0) {
    }
  }
  [3]=>
  object(common\models\Answer)#224 (10) {
    ["_attributes":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_oldAttributes":"yii\db\BaseActiveRecord":private]=>
    NULL
    ["_related":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_relationsDependencies":"yii\db\BaseActiveRecord":private]=>
    array(0) {
    }
    ["_errors":"yii\base\Model":private]=>
    NULL
    ["_validators":"yii\base\Model":private]=>
    NULL
    ["_scenario":"yii\base\Model":private]=>
    string(7) "default"
    ["_events":"yii\base\Component":private]=>
    array(0) {
    }
    ["_eventWildcards":"yii\base\Component":private]=>
    array(0) {
    }
    ["_behaviors":"yii\base\Component":private]=>
    array(0) {
    }
  }
}

Answer 1

well the problem is with your

echo  $count > 1?$form->field($answer, "[$index]answer")->checkboxList(ArrayHelper::map($model->questOptions, 'id_option', 'title_option'))->label('') : $form->field($answer, "[$index]answer")->radioList(ArrayHelper::map($model->questOptions, 'id_option', 'title_option'))->label('') ;
                                    ?>  

you need to set an attribute for each question on your models a thing like that

 [['answer','answer1','answer2','answer3','answer4'], 'string']