CODEWITHSUNDEEP
mongodbmapreduceaggregation-framework
Chris Webster
Chris Webster

Reputation: 918

MongoDB: convert array elements to fields

I am on MongoDB 2.6.0, and I have a collection with documents like this:

{ "mid" : 1021, 
  "day" : 298,
  "data":[
  {"ts" : 1,"kwh" : 0.017},
  {"ts" : 2,"kwh" : 0.018},
  {"ts" : 3,"kwh" : 0.019},
  ... ] }

I would like to flatten the array elements into individual fields like this:

{ "mid" : 1021, 
  "day" : 298,
  "ts1" : 0.017,
  "ts2" : 0.018,
  "ts3" : 0.019,
  ...
}

This looks like it should be possible with the Aggregation framework, but I really can't figure out how to re-project the data array's "kwh" elements based on the value of "ts".

Anybody know how to do this?

Thanks for any help!

Upvotes: 0

Views: 150

Answers (1)

Neil Lunn
Neil Lunn

Reputation: 151220

More of a mapReduce task really:

db.collection.mapReduce(
    function () {

      var copy = this;
      var tdata = copy.data;
      delete copy.data;

      tdata.forEach(function(data) {
        var key = "ts" + data.ts;
        copy[key] = data.kwh;
      });

      var id = copy._id;
      delete copy["_id"];

      emit( id, copy );

    },
    function(){},
    { "out": { "inline": 1 } }
)

At present you cannot flexibly specify the value of a "key" using the aggregation framework. You wold have to explicitly name each key through projection in order to get the output you want.

MapReduce does not give "exactly" the same output as you want do to how it works. But it is the closest you will get without explicitly naming the keys.

Upvotes: 1

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