count / group by query in xpdo?

SELECT categories, COUNT(*) AS cat_count FROM Box GROUP BY categories;

$query = $xpdo->newQuery('Box');
$query->select('id, category');
$query->groupBy('category');
$boxes = $xpdo->getCollection('Box',$query);  

foreach ($boxes as $row) {
$rows .= $row->get('category').$row->get('cat_count').'<br />';
}

echo $rows;