moving average with mongodb??

[Andrés]

Hello,

How can I to create a moving average with mongodb?

My documents have 2 fields: Month and visits

thx!

[Will Berkeley]

You didn't specify your data structure very well or give an example of exactly what you want, so I'm going to make it up to make it convenient. In the mongo shell, assuming documents of the form

    { "month": 0, "visits": 100 }

    { "month": 1, "visits": 200 }

    { "month": 2, "visits": 300 }

    { "month": 3, "visits": 400 }

    { "month": 4, "visits": 500 }

use an aggregation pipeline with input the current month

// return the average number of visits going back backMonth months from currentMonth (inclusive both ends)
movingAverage = function(currentMonth, backMonths) {
    var result = db.visitsByMonth.aggregate([
      { "$match" : { "month" : { "$lte" : currentMonth, "$gte" : currentMonth - backMonths } } },
      { "$group" : { "_id" : 0, "sum" : { "$sum" : "$visits" }, "count" : { "$sum" : 1 } } },
      { "$project" : { "average" : { "$divide" : ["$sum", "$count"] } } }
    ])
    return result.toArray()[0].average
}

This isn't a robust function but it has the essential ideas.

> movingAverage(4, 0)
500
> movingAverage(4, 1)
450
> movingAverage(4, 2)
400
> movingAverage(4, 3)
350
> movingAverage(4, 4)
300

-Will

[Andrés]

Will! Is perfect!!! Thank you soooo much!!!!

[César Antonio Pérez Quintana]

The last two elements of the pipeline can be reduced to this one:

{$group: {_id:0, average:{$avg: "$visits"}}}

[Will Berkeley]

Good point. I overlooked that operator.

-Will