ajax - Why I am getting error message when using Yii CActiveForm::validate() with array -

i have problem related cactiveform::validate(). have form , and sending data database using ajax, form contains multiple selectable drop-down list. in data saving section of controller produced following error

mb_strlen() expects parameter 1 string, array given (.../framework/validators/cstringvalidator.php:84) 

and after updating framework newer version, error gone, , got below validation message instead.

category ids invalid. 

if form filled(i mean rules in model satisfied), not produce such bug or error message.

controller action

public function actioncompany() {         $model = new company;         if (isset($_post['company'])) {             $model->attributes = $_post['company'];             $category_ids = "";             if (is_array($_post['company']['category_ids']))                 $category_ids = implode(',', $_post['company']['category_ids']);             $model->category_ids = $category_ids;             if ($model->validate()) {                 /*$temp = company::model()->findbypk($model->id);                 if ($temp !== null) {                     $model = $temp;                 }*/                 $model->save();                 echo cjson::encode(array('status' => 'success'));                 yii::app()->end();             } else {                 $error = cactiveform::validate($model);                 if ($error != '[]')                     echo $error;             }         }     } 

model rules

public function rules()     {          return array(             array('...., category_ids,...', 'required'),             array('..., category_ids, ...', 'length', 'max'=>255),             ....             .....             array('...., category_ids,...', 'safe', 'on'=>'search'),         );     } 

what i'm missing?

by default, cactiveform::validate($model) loads model attributes $_post , overrides current attribute values, destroying transformed values. pass false third argument avoid this.