Yii filter results without relation - php

Question

I am using CGridView in Yii which is listing results with some of relations from other models
<?php $this->widget('zii.widgets.grid.CGridView', array(
'dataProvider'=>$model->search(),
'filter' => $model,
'columns' => array(
array(
'name' => 'user.name',
'header' => Yii::t('app', 'User'),
'value' => '!isset($data->user->name) ? Yii::t("app", "Unknown user") : $data->user->name',
'type' => 'raw',
'filter' => CHtml::activeTextField($model, 'keyName'),
),
array(
'name' => 'visitor.company',
'header' => Yii::t('app', 'Company'),
'value' => 'isset($data->visit->visitor->company) ? $data->visit->visitor->company : (isset($data->rfid->rfid) ? Yii::t("app", "Company 1") : Yii::t("app", "Unknown company"))',
'type' => 'raw',
'filter' => CHtml::activeTextField($model, 'visitorCompany'),
),
array(
'name' => 'pass_id',
'header' => Yii::t('app', 'Pass'),
'value' => '!isset($data->pass_id) ? Yii::t("app", "Massing pass") : "<span class=\"label label-primary\">" . $data->pass_id . "</span>"',
'type' => 'raw',
),
),
)); ?>
and there is isset() condition which checks if relation exists (otherwise an error occurs)
But now I am looking for a way to filter those results which. For example to write "Unknown" in the filter to get the results without relations in the other table or "Company" to get those without some relation but with another existing relation