EMongoCriteria：限制和分组显示更少的行

I am using MongoDB, with PHP YII. I have used YiiMongoDbSuite for setting up the criteria for mongoDB Queries.

Currently, I am using Group by and Limit together. But due to some reason queries are returning less number of rows than that are expected.

$criteria=new EMongoCriteria();

$criteria->group('col_1'); 
$criteria->limit(10); 
$result = TableName::model()->findAll($criteria);

Can somebody guide me as I am quite new to MongoDB and YiiMongoDbSuite.

Thanks in advance,

Well to do it using MongoYii (which I maintain):

$result = MongoModel::model()->aggregate(
    array(
        '$group' => array('_id' => 'col_1'), 
        '$limit' => 10
    )
)

I am unsure how to do it with YiiMongoDbSuite, in fact there is no group command in its EMongoCriteria from what I see.