Yii框架下MongoDB如何查询单个Collection下所有数据的单个字段？

$criteria = new EMongoCriteria();
$criteria->sort('posts_count',EMongoCriteria::SORT_DESC)
->limit(25)
->offset(0);
$test_id = Group::model()->findAll($criteria);
$test_id->next();
$_ids = array();
$name_id = array();
while($row = $test_id->current()){
$t = $row->attributes;
$name_id[] = array_splice($t,0,2);
$_ids[] = $name_id;
$test_id->next();
}
$test_id = $_ids[count($_ids)-1];