The total size of the messages - mapReduce

Create a mapping function:


map = function() { emit('msize', this.size); }

Create a reduce function:


red = function(k, v) {
  return Array.sum(v);
}

Run the mapReduce:


res = db.messages.mapReduce(map, red, {out: "Result"})

Get the result from the temporary collection called res.


res.find()

{
  "_id": "msize",
  "value": 17318623
}