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
}