Calculating Fields In A Relational Query

I am pounding my head on the desk trying to get calculated results from a HAS_MANY relation. Specifically I want to add quantities together

in model:

	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(
			'submissionitems' => array(self::HAS_MANY, 'RebateSubmissionItems', 'RebateProductID'),
			
		);
	}

In my controller:

		$criteria = new CDbCriteria;
		$criteria->select = 'Product,UPC, StartDate, EndDate, DeadlineDate,ProductID';
		$criteria->with = array('submissionitems'=>array('select'=>"sum('submissionitems.Quantity') as TotalRebates"));

in my view:

$this->widget('zii.widgets.grid.CGridView', array(
	'id'=>'rebate-products-grid',
	'dataProvider'=>$dataProvider,
	'filter'=>$model,
	'columns'=>array(
		array(
	        'class'=>'CLinkColumn',
	        'labelExpression'=>'$data->Product',
	        'urlExpression'=>'"../rebateSubmissionItems/indexRebate?thisproduct=".$data->Product."&thisrebate=".$data->ProductID',
	        'header'=>'Rebate Product'
	      ),
		'UPC',		
		'StartDate',
		'EndDate',
		'DeadlineDate',
		'TotalRebates'
	),
));

The TotalRebates column is empty. Any ideas where I went wrong??