Mongo mapreduce - use $dateToString on mapping stage

Question

I have simple collection of elements with following structure:

{
   _id: MongoId
   pvs: int
   day: IsoDate()
   uid: int
}

I'd like to use MapReduce to count pageviews for given user, grouped by certain date range (day/week/month, date format compatible).

What I'm stuck is how to reformat IsoDate in map function using $dateToString before emitting, so it emmits the format I want, like %Y-%m-%d or %Y-%m or %Y-%m-%U. When I'm calling it, I'm not getting reformatted date, but object with format and date fields.

Example:

function(){
    emit(
        {'$dateToString': {'format': "%Y-%m-%d", 'date': this.day}}, 
        this.pvs
    )}

will return

{
    "pvs" : 5
    "$dateToString" : {
        "format" : "%Y-%m-%d",
        "date" : ISODate("2016-07-13T08:27:29.000Z")
    }
}

I want to have this returned instead:

{
    "pvs": 5,
    "day": "2016-07-13"
}

Answer 1

If using mapReduce then you would have to create your own custom function that formats the date and call that in your map function:

dateToString = function(date){
    return date.getFullYear() + '-' (date.getMonth() + 1) + '-' + date.getDate();
}

map = function() {
    emit(dateToString(this.day), this.pvs);
}

---

Better with the aggregation framework which runs "within" MongoDB in its C++ code hence more efficient that mapReduce which runs within a V8/spidermonkey (depending on your version) environment within the bundled JS console:

db.collectionName.aggregate([
    { "$match": { "uid": userId } },
    { 
        "$project": {
            "formattedDate": { 
                "$dateToString": { "format": "%Y-%m-%d", "date": "$day" } 
            },
            "pvs": 1
        }
    },
    {
         "$group": {
             "_id": "$formattedDate",
             "pvs": { "$sum": "$pvs" }
         }
    }
])

which in doctrine mongo odm you can run your pipeline using the command function as:

$connection = $this->get('doctrine_mongodb')->getConnection();
$mongo = $connection->getMongo();
if (!$mongo) {
    $connection->connect();
    $mongo = $connection->getMongo();
}
$db = $mongo->selectDB('test_database');
$aggregate_results = $db ->command(array( 
    "aggregate" => "collectionName",
    "pipeline" => array( 
        array("$match" => array("uid"=>  userId )),
        array( 
            "$project" => array(
                "formattedDate" => array( 
                    "$dateToString" => array("format" => "%Y-%m-%d", "date"=>  "$day") 
                ),
                "pvs" =>  1
            )
        ),
        array(
             "$group" => array(
                 "_id" => "$formattedDate",
                 "pvs" => array("$sum" => "$pvs")
             )
        )
    )
));