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 function should emit a timestamp object, adjusted up to the minute, and a count of 1. The reduce function 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};
}