Taking sum of "column" in MongoDB

Question

In MySQL, I would simply go "SELECT sum(pts) FROM table" to get the sum of the pts column on the table. However, I am converting this application to MongoDB and cannot find a straightforward way to get a sum of the common key "type" in my collection. I am using PHP, so please give PHP code to make this work.

Here is my data:

{ "_id" : ObjectId("4f136dab5056f65b61000000"), "p_id" : "3", "g_id" : "1", "type" : "3" }
{ "_id" : ObjectId("4f136dad5056f65760000000"), "p_id" : "8", "g_id" : "1", "type" : "7" }
{ "_id" : ObjectId("4f136daf5056f65860000000"), "p_id" : "6", "g_id" : "1", "type" : "4" }
{ "_id" : ObjectId("4f136db15056f65460000000"), "p_id" : "27", "g_id" : "1", "type" : "2" }

How can I get $sum to equal the total of the "type" key?

Answer 1

Utilizing MongoDB's new Aggregation Framework:

$result = $db->command(array(

    'aggregate' => 'COLLECTION_NAME',

    'pipeline'  => array(
        // Match:
        // We can skip this in case that we don't need to filter 
        // documents in order to perform the aggregation
        array('$match' => array(
            // Criteria to match against
        )),

        // Group:
        array('$group'  => array(
            // Groupby fields:
            '_id'       => "$fieldname1, $fieldname2, $or_empty, $etc",
            // Count:
            'count'     => array('$sum' => 1),
            // Sum:
            'type_sum'  => array('$sum' => '$type'),
        ))

        // Other aggregations may go here as it's really a PIPE :p

    ),
));