MongoDB keeps giving me null - mongodb

Question

I have some difficulty in getting Mongodb aggregate to work. It keeps giving me null. Please help. Below are the codes written in php. Thanks.
What I want to do is to sum up the values of 2 fields, Requests and Responses, between 2 particular dates
try {
$mongodb = new MongoClient("mongodb://ad:pass2word1#localhost");
$database = $mongodb->selectDB('backend');
$collection = new MongoCollection($database, 'RequestSummary');
$pipeline = array(
array(
'$group' => array(
'_id' => array(
'request' => array('$sum' => '$Requests'),
'response' => array('$sum' => '$Responses')
)
)
),
array(
'$match' => array(
'RequestDate' => array(
'$gte' => intval($_SESSION['range_from']),
'$lte' => intval($_SESSION['range_to'])
)
)
)
);
$collection->aggregate($pipeline);
var_dump($g);
} catch (MongoConnectionException $exc) {
echo $exc->getTraceAsString();
}

Answer 1

The _id of your $group can't contain aggregation operators like $sum. Those sums need to be defined as fields at the same level as _id. If you don't want to group on a specific field you can use NULL for the _id like this:
array(
'$group' => array(
'_id' => NULL,
'request' => array('$sum' => '$Requests'),
'response' => array('$sum' => '$Responses')
)
),