// View file

<?php echo CHtml::dropDownList(
                        'topcat',
                        CHtml::listData(
                             Categories::model()->findAll('parent_id=0'),
                            'id',
                            'title'
                         ),
                        array(
                            'ajax' => array(
                                'type' => 'POST',
                                'url' => CController::createUrl('/products/subs'),
                                'update' => '#sub',
                            )
                        ),
                        array('empty'=>'Επιλέξτε Κατηγορία')
                    );
        ?>
        <div id="sub">
            <?php
                echo CHtml::dropDownList(
                   $model, 'category_id', '', array()
                );
            ?>
        </div>




// ProductsController.php

public function actionSubs()
    {
        $data = Location::model()->findAll('parent_id=:parent_id', array(':parent_id' => (int)$_POST['topcat']));

        $data = CHtml::listData($data, 'id', 'title');
        foreach($data as $value=>$name)
        {
            echo CHtml::tag('option', array('value' => $value), CHtml::encode($name), true);
        }
    }