public function relations()
{
        return array(
                'category' => array(self::BELONGS_TO, 'Category', 'category_id'),
        );
}

<div class="row">
        <?php echo $form->labelEx($model,'category_id'); ?>
        <?php echo $form->dropDownList($model,'category_id',???); ?>
        <?php //echo $form->textField($model,'category_id'); ?>
        <?php echo $form->error($model,'category_id'); ?>
</div>

<?php
$list = CHtml::listData(Category::model()->findAll(array('order' => 'name')), 'id', 'id'));
    echo $form->dropDownList($model,'category_id',$list);
?>