Displaying value from join table in Yii CgridView - yii

Question

I have this very simple table in my view and I'm trying to display value from joined table:
$this->widget('zii.widgets.grid.CGridView', array(
'id'=>'skills-grid',
'dataProvider'=>$model->searchWithStudentSuccessRate($id),
'template' => '{items}{pager}',
'cssFile'=>Yii::app()->request->baseUrl. '/themes/'. Yii::app()->theme->name.'/css/table.css',
'htmlOptions'=>array('class'=>'datagrid', 'style'=>'width:550px;'),
'columns'=>array(
array(
'name'=>'name',
),
array(
'name' => 'value',
'header' => Yii::t('MainTrans', 'Value'),
'value' => '$data->student_skills->value',
),
array(
'name' => 'successRate',
'header' => Yii::t('MainTrans', 'Success Rate'),
'value' => '$data->successRate."%"',
),
),
));
And this is the search function:
public function searchWithStudentSuccessRate($id)
{
// Warning: Please modify the following code to remove attributes that
// should not be searched.
$criteria=new CDbCriteria;
$criteria->compare('id',$this->id);
$criteria->compare('name',$this->name,true);
$criteria->compare('t.threshold',$this->threshold);
$criteria->with = array('student_skills');
$criteria->together = true;
$criteria->select = array('IFNULL(CASE WHEN ROUND((student_skills.value/t.threshold)*100,0) > 100 THEN 100 ELSE ROUND((student_skills.value/t.threshold)*100,0) END,0) as successRate','*');
$criteria->group = "t.name";
$criteria->condition = 'student_skills.student_id = '.$id;
$criteria->compare('successRate',$this->successRate);
$criteria->compare('student_skills.value',$this->value);
return new CActiveDataProvider($this, array(
'criteria'=>$criteria,
'pagination'=>array(
'pageSize'=>25,
),
'sort'=>array(
'defaultOrder'=>array(
'successRate'=>CSort::SORT_DESC,
),
'attributes'=>array(
'successRate'=>array(
'asc'=>'successRate',
'desc'=>'successRate DESC',
),
'*',
),
),
));
}
But I get error: "Trying to get property of non-object" when I added value column to my CGridView.
Everything works fine without column value (columns successRate and name are fine). Relation should be fine as well. The error I get means that the value doesn't exist but it should since I did something similar in my other views and there it works.
Can anyone tell what's wrong? I'm sure it's something minor but I'm struggling with this embarrasing problem for a while.
Thanks

Answer 1

It means that in some conditions $data->student_skills is NULL. Try change this:
'value' => '$data->student_skills->value',
to this
'value' => 'empty($data->student_skills) ? null : $data->student_skills->value',