HHVM MongoDb aggregation seems like it doesn't work - php

Question

I have some php script running on hhvm
I'm trying to get max value of numeric field from my collection in MongoDB 3.2
Here is my aggregation pipeline
$mongo = new \MongoDB\Driver\Manager(MONGODB_HOST);
$myCollection = new \MongoDB\Collection($mongo, "mydb.mycollection");
$pipeline = [
[
'$group' => [
'_id' => 'group_field',
'slId' => ['$max' => '$saleId']
],
]
];
$doc = $myCollection->aggregate($pipeline);
This pipeline perfectly works in mongo shell, but from php $doc contains all documents from my collection and no $group is applied to them
Maybe someone can help me with that?

Answer 1

The aggregation operation is returning all documents from your collection since you are specifying a constant value for the group by key, the string group_field. You need to prefix the group field with the $ character in your _id key value. So for example if your group by key is the name field, you can rewrite the aggregation pipeline as
$mongo = new \MongoDB\Driver\Manager(MONGODB_HOST);
$myCollection = new \MongoDB\Collection($mongo, "mydb.mycollection");
$group_field = '$name';
$pipeline = [
[
'$group' => [
'_id' => $group_field,
'slId' => ['$max' => '$saleId']
],
]
];
$doc = $myCollection->aggregate($pipeline);

Answer 2

Thanks to chridam's answer up here.
I don't know, but maybe this is some kind of magic
So, chridam's answer helped me with $group, so finally my results were grouped by $group_field. But slId still wasn't there. So, here is what i did and it helped:
$f = '$group_field';
$sl = ['$max' => '$saleId'];
$pipeline = array(
array(
'$group' => array(
'_id' => $f,
'slId' => $sl
),
)
);