Question

We’ve setup a basic application which writes events to a mongo instance. Here’s a sample write:

"_id" : ObjectId("50fee761472870a3d610956e"),
    "user_id" : "pa-XXXXXXXXX",
    "event_id" : 1,
    "date_created" : 1337798856,
    "url" : "http://news.yahoo.com/australian-tycoon-worlds-richest-woman-103810206.html"

What we’d like to do is retrieve results grouped by URL and a count of certain event_id’s. Like event_grouping_a might consist of event_ids 1,6,35 and event_grouping_b might consist of 2,66,103. A sample output would look something like this:

{
    url: "http://news.yahoo.com/australian-tycoon-worlds-richest-woman-103810206.html",
    event_grouping_a: 46,
    event_grouping_b: 34
},
{
    url: "http://news.yahoo.com/another-cool-story",
    event_grouping_a: 105,
    event_grouping_b: 59
}

Any idea on how to perform this type of aggregation/grouping? The end goal is to have it in PHP, but I’ve messed with doing it in the mongod console to no avail. I can get it to group by URL, but I can’t get it do display both event types under a single parent URL. It spits out something like this:

{
    url: "http://news.yahoo.com/australian-tycoon-worlds-richest-woman-103810206.html",
    event_grouping_a: 46
    event_grouping_b: 0
},
{
    url: "http://news.yahoo.com/australian-tycoon-worlds-richest-woman-103810206.html",
    event_grouping_a: 0
    event_grouping_b: 34
},
...

The above two should be merged into 1, but I can’t for the life of me figure it out … any suggestions?

Answer 1

I don’t necessarily like the $or statements to make this work, but this query should work for you from php if you are using the 1.3+ mongo driver:

$mongo = new MongoClient('dbinfo');
$collection = $mongo->selectCollection( 'dbName', 'collectionName' );

$pipeline = array(
    array( 
        '$group' => array(
            '_id' => '$url',
            'event_grouping_a' => array( 
                '$sum' => array( 
                    '$cond' => array( 
                        array( '$or' => array(
                            array( '$eq' => array( '$event_id', 1 ) ),
                            array( '$eq' => array( '$event_id', 6 ) ),
                            array( '$eq' => array( '$event_id', 35 ) )
                        ) ), 1, 0 ) 
                )   
            ),
            'event_grouping_b' => array(
                '$sum' => array(
                    '$cond' => array(
                        array( '$or' => array(
                            array( '$eq' => array( '$event_id', 2 ) ),
                            array( '$eq' => array( '$event_id', 66 ) ),
                            array( '$eq' => array( '$event_id', 103 ) )
                        ) ), 1, 0 )
                )
            )
        )
    )
);
$results = $collection->aggregate( $pipeline );

—

var_dump( $results );
array(2) { 
    ["result"]=> array(2) { 
        [0]=> array(3) { 
            ["_id"]=> string(40) "http://news.yahoo.com/another-cool-story" 
            ["event_grouping_a"]=> int(2) 
            ["event_grouping_b"]=> int(1) 
        } 
        [1]=> array(3) { 
            ["_id"]=> string(75) "http://news.yahoo.com/australian-tycoon-worlds-richest-woman-103810206.html" 
            ["event_grouping_a"]=> int(3) 
            ["event_grouping_b"]=> int(4) 
        } 
    } 
    ["ok"]=> float(1) 
}