Map Reduce count number of documents in each minute MongoDB

Question

I have a MongoDB collection which has a created_at stored in each document. These are stored as a MongoDB date object e.g.

{ "_id" : "4cacda7eed607e095201df00", "created_at" : "Wed Oct 06 2010 21:22:23 GMT+0100 (BST)", text: "something" }
{ "_id" : "4cacdf31ed607e0952031b70", "created_at" : "Wed Oct 06 2010 21:23:42 GMT+0100     (BST)", text: "something" }
....

I would like to count the number of items created between each minute, so I can pass the data into Google Charts to generate something like this:

How do I do this with a map reduce function, or is there a fancy mongodb aggregate function which I could use instead?

Answer 1

Map should emit a timestamp object, adjusted up to the minute, and a count of 1. The reduce should sum all the counts.

map = function() {

var created_at_minute = new Date(this.created_at.getFullYear(),
                                 this.created_at.getMonth(), 
                                 this.created_at.getDate(), 
                                 this.created_at.getHours(), 
                                 this.created_at.getMinutes());
    emit(created_at_minute, {count: 1});
}

reduce = function(key, values) { 
         var total = 0;
         for(var i = 0; i < values.length; i++) { total += values[i].count; }
         return {count: total};
}

Answer 2

Hi Tom! You can also try group function.

db.stat.group({key:{"create_at_minute":true}
              ,initial:{count:0}
              ,reduce:function(doc,out){out.count++}})

where create_at_minute is your create_at field rounded by minutes.