skipped count 0 in aggregate function

Question

I\'m stuck on this for couple of days. I\'m trying to get the count: 0 where there is no documents in the given time period. This is the aggregate function I\'m

Answer 1

this is hacky fix I did for now:

var getCount = function(timeBlock, start, end, cb) {

    Document.aggregate(
    {
        $match: {
            time: {
                $gte: new Date(start),
                $lt: new Date(end)
            }
        }
    },

    {
        $project: {
            time: 1,
            delta: { $subtract: [
                new Date(end),
                '$time'
            ]}
        }
    },

    {
        $project: {
            time: 1,
            delta: { $subtract: [
                "$delta",
                { $mod: [
                    "$delta",
                    timeBlock
                ]}
            ]}
        }
    },

    {
        $group: {
            _id: { $subtract: [
                end,
                "$delta"
            ]},
            count: { $sum: 1 }
        }
    },

    {
        $project: {
            time: "$_id",
            count: 1,
            _id: 0
        }
    },

    {
        $sort: {
            time: 1
        }

    }, function(err, results) {
        if (err) {
            cb(err)
        } else {
            // really hacky way
            var numOfTimeBlocks = ( end - start ) / timeBlock

            // in case there is no 0s in the given period of time there is no need
            // to iterate through all of the results
            if ( results.length === numOfTimeBlocks ) {
                cb(results);

            } else {
                var time = start;
                var details = [];

                var times = results.map(function(item) {
                    return item.time;
                });

                for( var i = 0; i < numOfTimeBlocks; i++) {
                    time += timeBlock;
                    var idx = times.indexOf(time);

                    if (idx > -1) {
                        details.push(results[idx]);
                    } else {
                        var documentCount = { count: 0, time: time };
                        details.push(documentCount);
                    }
                }

                cb(details);
            }

        }
    })
}

I was also thinking about doing one query per time block, which gives the same result but I think is inefficient because you query the database N times.