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Advanced data aggregation: average in MongoDB collections

I have a set of documents like:

{
"browser" : "firefox",
"version" : "4.0.1"
}

{
"browser" : "firefox",
"version" : "3.6.2"
}

{
"browser" : "ie",
"version" : "8.0"
}

How to calculate the average value for all browsers so that the results are as follows:

global firefox: 66%
global ie: 33%

precise firefox:
4.0.1: 50%
3.6.3: 50%

The tricky part is that I don't want to provide all versions of Firefox available in the array. The MongoDB query should find all the individual versions in the collection and calculate the average value for all of them.

Thanks in advance!

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1 answer

Here is a solution that produces your statistics with clean numbers (e.g. 0.5 instead of 50%):

var m = function() {
  emit('global', this.browser);
  emit('local', [this.browser, this.version]);
};

var r = function(key, values) {
  var global={}, local={}, total=0, i, j, x;
  if (key == 'global') {
    values.forEach(function(v) {
      global[v] = (global[v]||0) + 1;
      total += 1;
    });
    for (i in global) { global[i] = global[i] / total; }
    return global;
  } else if (key == 'local') {
    values.forEach(function(v) {
      if (!local[v[0]]) { local[v[0]] = {}; }
      x = local[v[0]];
      x[v[1]] = (x[v[1]]||0) + 1;
    });
    for (i in local) {
      total = 0;
      x = local[i];
      for (j in x) { total += x[j]; }
      for (j in x) { x[j] = x[j] / total; }
    }
    return local;
  };
};

db.browsers.mapReduce(m, r, {out:'bout'});
db.bout.find();
// => { "_id" : "global", "value" : { "firefox" : 0.6666666666666666, "ie" : 0.3333333333333333 } }
// => { "_id" : "local", "value" : { "firefox" : { "4.0.1" : 0.5, "3.6.2" : 0.5 }, "ie" : { "8.0" : 1 } } }
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