Question

I have data in the form

{userid: 123,
actiontype: 'loggedin',
timestamp: date-time}

I want to run a query which will group the data for unique combinations of userid and actiontype, and return the document with the latest timestamp value.

Any ideas how I can do this?
At present i am thinking of doing a find and then throwing the returned values into arrays (python and pymongo) and then do the operations there. However, i feel that it will be much more elegant/efficient to do this directly on the mongodb.

Answer 1

You can do it with Mongo group function. The following code shows how you can do it.

db.stack.group(
{
    keyf: function(doc) {
        return {
            userid : doc.userid, 
            actiontype : doc.actiontype
        };
    },
    reduce: function(obj,prev) {
        if (prev.maxdate < obj.timestamp) { 
            prev.maxdate = obj.timestamp; 
        } 
    },
    initial: {maxdate:0}
});