Yii receiving data from checkboxlist

Question

I have some project in Yii. First it was a form with two text fields: Name and Subject (subject is list of subjects separated by comma). Now I need to replace subject text field with list of checkboxes, which will add the same string separated by comma.

<!--<?php echo $form->labelEx($model,'Subjects'); ?>
<?php echo $form->textField($model,'Subjects',array('size'=>60,'maxlength'=>255)); ?>
<?php echo $form->error($model,'Subjects'); ?>-->
<?php echo $form->labelEx($model,'Subjects'); ?>
<?php echo $form->checkBoxList($model,'Subjects',$this->listOfSubjects()); ?>
<?php echo $form->error($model,'Subjects'); ?>

List of subjects is a controller method that returns array required for checkboxlist in Yii, something like:

array(
'1'=>'Something',
'2'=>'Anotherthing'
);

Also here is code of my action create. It's rather standart:

public function actionCreate()
{
    $model=new CrdTeachers;
    if(isset($_POST['CrdTeachers']))
    {
        $model->attributes=$_POST['CrdTeachers'];
        if($model->save())
            $this->redirect(array('view','id'=>$model->Teacher_ID));        
    }
    $this->render('create',array(
        'model'=>$model,
    ));
}

This code worked fine when there was just two text fields. Now when I'm using create action it says that mistake:

mb_strlen() expects parameter 1 to be string, array given

I can't find where can I process it's data to make it string. Any experts of Yii here? What should I look for?
UPDATE: Here is CRDTeachers model class

class CrdTeachers extends CActiveRecord
{
    /**
     * Returns the static model of the specified AR class.
     * @param string $className active record class name.
     * @return CrdTeachers the static model class
     */
    public static function model($className=__CLASS__)
    {
        return parent::model($className);
    }

    /**
     * @return string the associated database table name
     */
    public function tableName()
    {
        return 'crd_teachers';
    }

    /**
     * @return array validation rules for model attributes.
     */
    public function rules()
    {
        // NOTE: you should only define rules for those attributes that
        // will receive user inputs.
        return array(
            array('Name, Subjects', 'required'),
            array('Name, Subjects', 'length', 'max'=>255),
            // The following rule is used by search().
            // Please remove those attributes that should not be searched.
            array('Teacher_ID, Name, Subjects', 'safe', 'on'=>'search'),
        );
    }

    /**
     * @return array relational rules.
     */
    public function relations()
    {
        // NOTE: you may need to adjust the relation name and the related
        // class name for the relations automatically generated below.
        return array(
            //'rSubjects'=>array(self::HAS_MANY, 'CrdSubjects', 'Subject_ID'),
            //'categories'=>array(self::MANY_MANY, 'CrdTeachers', '{{CrdSubjects}}(Subject_ID, Subject_Name)'),
        );
    }

    public function behaviors()
    {
        return array(
            'DMultiplyListBehavior'=>array(
                 'class'=>'DMultiplyListBehavior',
                 'attribute'=>'categoriesArray',
                 'relation'=>'categories',
                 'relationPk'=>'id',
            ),
        );
    }

    protected function afterSave()
    {
        //$this->refreshCategories();
        parent::afterSave();
    }

    protected function refreshCategories()
    {
        $categories = $this->categoriesArray;

        CrdTeachers::model()->deleteAllByAttributes(array('Subject_ID'=>$this->id));

        if (is_array($categories))
        {
            foreach ($categories as $id)
            {
                if (Category::model()->exists('id=:id', array(':id'=>$id)))
                {
                    $postCat = new CrdTeachers();
                    $postCat->post_id = $this->id;
                    $postCat->category_id = $id;
                    $postCat->save();
                }
            }
        }
    }   

    /**
     * @return array customized attribute labels (name=>label)
     */
    public function attributeLabels()
    {
        return array(
            'Teacher_ID' => 'Teacher',
            'Name' => 'Name',
            'Subjects' => 'Subjects',
        );
    }

    /**
     * Retrieves a list of models based on the current search/filter conditions.
     * @return CActiveDataProvider the data provider that can return the models based on the search/filter conditions.
     */
    public function search()
    {
        // Warning: Please modify the following code to remove attributes that
        // should not be searched.

        $criteria=new CDbCriteria;

        $criteria->compare('Teacher_ID',$this->Teacher_ID);
        $criteria->compare('Name',$this->Name,true);
        $criteria->compare('Subjects',$this->Subjects,true);

        return new CActiveDataProvider($this, array(
            'criteria'=>$criteria,
        ));
    }

    /*
    * Возвращает список по ID
    */
    public function getParentTypeById($id) {
        $title = $this->model()->findByPk($id)->Name;
        return $title;
    }   
}

Answer 1

You may use this method in your model.

public function beforeValidate() 
{
    if (is_array($this->Subjects)) {
        $this->Subjects = implode(', ', $this->Subjects);
    }

    return parent::beforeValidate();
}

Answer 2

You can process the input before validation with the beforeValidate function in the model. Make sure that you run the parent method and return true if want validation to continue. Usually it is done like this:

public function beforeValidate() 
{
    // do stuff to transform the array into the string
    return parent::beforeValidate();
}