Hide CGridview Delete Button If the Record is used

In my project, I have to hide the delete button from 'CGridview' If the record is not related to any other records or table and i have to check the record id, Is it possible or not to delete. Now i created one function 'is_used()' in model. This function check this record 'ID' is existed in relation table. If it is existed, it will return TRUE otherwise we will get FALSE statement.

After created this function, i will call it from 'CGridview' widget for every record. See the below code to run this concept.

User Model

<?php
class User extends CActiveRecord
{
    ....................
    /**
    * @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(
           'userComments' => array(self::HAS_MANY, 'UserComments', 'user_id'),
           'user' => array(self::HAS_MANY, 'User', 'createdby'),
        );
    }
	// we have to call this from CGridview
    public function is_used()
    {
        if($this->userComments)
            return true;
        else if($this->user)
            return true;
        else
            return false;
    }
    ....................
}
?>

User CGridView

<?php 
$this->widget('zii.widgets.grid.CGridView', array(
    'id'=>'user-grid',
    'dataProvider'=>$model->search(),
    //'filter'=>$model,
    'columns'=>array(
        ......
        'username',
        ......
        array(
            'class'=>'CButtonColumn',
            'buttons'=>array(
                'delete'=>array(
                    'visible'=>'$data->is_used()?false:true'
                ),
            ),
        ),
    ),
)); ?>

Delete Action In User Controller

<?php 
class UserController extends Controller
{
    ...............
    public function actionDelete($id)
    {
        $model=$this->loadModel($id);
        if($model->is_used()==false)
            $model->delete();
        else
           throw new CHttpException(404,'We cant delete this record.');

        if(!isset($_GET['ajax']))
            $this->redirect(isset($_POST['returnUrl']) ? $_POST['returnUrl'] : array('admin'));
    }
    ...............
}
 ?>