在monogdb的聚合和管道中使用where子句

Iam grouping a field, now I want to add where clause.
How to add where clause in following?

$query=array(
    'aggregate'=>$this->table_name,
    'pipeline'=> array(
        array('$group'=> array('_id'=> array(
                    'name'=>'$name',
                    "{$field}"=>'$'.$field
                ))),
        array('$group'=> array(
                '_id'=>'$_id.'.$field,
                'count'=> array('$sum'=>1)
            ))
    )
);
$result=$mongo_db->command($query);

you can use $match operator while using aggregation

http://docs.mongodb.org/manual/reference/operator/aggregation/match/