$criteria=new CDbCriteria;
$criteria->with = array(
'historicalUsageAvg'=>array
	(
		'order'=>'historicalUsageAvg.EnrollmentCompleted DESC',
		'limit'=>1,
		'select'=>'(AVGUsage*12) AS HistoricalUsageAvg'
	)
);
$criteria->together = true;

It only produces this, which is the id of the relation table
--

[historicalUsageAvg] => HistoricalUsage Object
                        (
                            [_new:CActiveRecord:private] =>
                            [_attributes:CActiveRecord:private] => Array
                                (
                                    [id] => 10340
                                )

                            [_related:CActiveRecord:private] => Array
                                (
                                )

                            [_c:CActiveRecord:private] =>
                            [_pk:CActiveRecord:private] => 10340
                            [_alias:CActiveRecord:private] => t
                            [_errors:CModel:private] => Array
                                (
                                )

                            [_validators:CModel:private] =>
                            [_scenario:CModel:private] => update
                            [_e:CComponent:private] =>
                            [_m:CComponent:private] =>
                        )