Sorting Error while using Computational Field

public function rules() {
        return array(
            array('name, email, subject, message', 'required', 'message' => Yii::t('app', 'MSG_ATTRIBUTE_BLANK')),
            array('email', 'email'),
            array('verifyCode', 'CaptchaExtendedValidator', 'allowEmpty' => !CCaptcha::checkRequirements(), 'on' => 'fContact'),
            array('name, email,subject,message,lastactivity', 'safe', 'on' => 'search'),
            array('name, subject, email, message, status,createdon,updatedon,verifyCode,lastactivity', 'safe'),
        );
    }
 public function relations() {
        return array(
            'ca' => array(self::HAS_MANY, 'ContactAction', 'contact_id'),
    );
}

public function search() {
        $criteria = new CDbCriteria;
        $lastactivity_sql = '(select max(date) from tbl_contact_action where contact_id=t.contact_id)';
        $criteria->select = array('t.*', $lastactivity_sql . ' as lastactivity');
        $criteria->addCondition('t.status<>"Deleted"');
        $criteria->compare($lastactivity_sql, $this->lastactivity);
        $sort = new CSort;
        $sort->defaultOrder = array('name' => CSort::SORT_ASC);
        $sort->attributes = array(
            'name' => 'name',
            'email' => 'email',
            'status' => 'status',
            'createdon' => 'createdon',
            'lastactivity' => 'lastactivity',
        );
        $sort->applyOrder($criteria);

        return new CActiveDataProvider($this, array(
            'criteria' => $criteria,
            'sort' => $sort,
            'pagination' => array(
                'pageSize' => Yii::app()->user->getState('contactPageSize', Yii::app()->params['RECORDPERPAGE_ADMIN']),
                'currentPage' => Yii::app()->user->getState('Contact_page', 0),
            ),
        ));
    }
CGRIDVIEW on display page
---------------------------
<?php
        $this->widget('ext.selgridview.SelGridView', array(
            'id' => 'contact_grid',
            'dataProvider' => $dataProvider,
            'pageSizeVar' => 'contact',
            'columns' => array(
        array(
                    'id' => 'autoId',
                    'class' => 'gridCheckBoxColumn',
                    'headerHtmlOptions' => array('class' => 'checkbox paracheck'),
                    'htmlOptions' => array('class' => 'checkbox paracheck'),
                ),
                array(
                    'header' => 'Last Activity',
                    'class' => 'gridDataColumn',
                    'name' => 'lastactivity',
                    'value' => 'CommonFunctions::formatDateTime($data->lastactivity)',
                    'htmlOptions' => array('class' => 'maincell'),
                    'headerHtmlOptions' => array('title' => Yii::t('app', 'LBL_LIST_SORT_DATE_TITLE'))
                ),
            ),
        ));
        ?>


Error
------
CDbCommand failed to execute the SQL statement: SQLSTATE[42S22]: Column not found: 1054 Unknown column 't.lastactivity' in 'order clause'. The SQL statement executed was: SELECT t.*, (select max(date) from tbl_contact_action where contact_id=t.contact_id) as lastactivity FROM `tbl_contact` `t` WHERE t.status<>"Deleted" ORDER BY lastactivity, `t`.`lastactivity` LIMIT 10