/**
  * Creates data provider instance with search query applied
  *
  * @param array $params
  *
  * @return ActiveDataProvider
  */
 public function search($params)
 {
     $query = Turma::find();
     $dataProvider = new ActiveDataProvider(['query' => $query]);
     $this->load($params);
     if (!$this->validate()) {
         // uncomment the following line if you do not want to return any records when validation fails
         // $query->where('0=1');
         return $dataProvider;
     }
     $query->andFilterWhere(['id' => $this->id]);
     $query->andFilterWhere(['like', 'descricao', $this->descricao]);
     return $dataProvider;
 }
use yii\helpers\ArrayHelper;
use app\models\Turma;
use yii\widgets\ActiveForm;
/* @var $this yii\web\View */
/* @var $model app\models\Avaliacao */
/* @var $form yii\widgets\ActiveForm */
?>

<div class="avaliacao-form">

    <?php 
$form = ActiveForm::begin(['method' => 'GET', 'action' => \yii\helpers\Url::to(['avaliacao/avaliar'])]);
?>

    <?php 
echo $form->field($turma, 'id')->dropDownList(ArrayHelper::map(Turma::find()->all(), 'id', 'descricao'), ['prompt' => '---- Turmas ----', 'name' => 'turma'])->label('Selecione a sua turma');
?>



    <div class="form-group">
        <?php 
echo Html::submitButton('Avaliar', ['class' => 'btn btn-primary']);
?>
    </div>

    <?php 
ActiveForm::end();
?>

</div>