Mongo and Pivot

Question

I need help with mongo in this problem: I have collection stats (UserId, EventId, Count, Date) in collection are data

UserID | EventId | Count | Date

Answer 1

You have to do this with a MapReduce operation.

Your map function would look like this: (untested!):

var mapFunction = function() {
                   var ret = {};
                   ret["Count_Event_" + this.EventId] = this.Count;
                   emit(this.UserId, ret);
               };

This emits a series of pairs consisting of the UserId and an object with a single, differently-named attribute with the count as a value.

Your reduce function would then combine the results into one (untested - I am not sure if you can just increment a non-existing property and I can't test it right now):

var reduceFunction = function(UserId, values_array) {
                   var ret = {};

                   for (var i = 0; i < values_array.length; i++) {
                       var values = values_array[i];
                       for (key in values) {
                           ret[key] += values[key]; // Can you increment a non-existing attribute? Not sure, try it, please.
                       }
                   }                       

                   return ret;
               };

You then call this like this:

 db.yourCollection.mapReduce(
                 mapFunction,
                 reduceFunction,
                 out: { inline: 1 }
               )

The line out: { inline: 1 } outputs the results into the console. Usually you use MapReduce to create a new collection with the results. See the documentation for more information.