Question

I have a bunch of documents:

{"timestamp" : 1304535236, "value" : 2}
{"timestamp" : 1304532236, "value" : 5}
{"timestamp" : 1304535647, "value" : 1}

where timestamp is a unix timestamp. I would like to sum up the total number of value for each day. Note: There may be a number of documents whose timestamps fall on the same day just at different times.

If I do a normal mongo group query is sums up per timestamp because they are all unique, which is un-helpful.

Answer 1

You have to convert your timestamp values into Date objects so that you can group by day.

Using Rachel’s answer as a starting point (in the shell):

db.test.group({
  keyf: function(doc) {
    var date = new Date(doc.timestamp*1000);
    var dateKey = (date.getMonth()+1) + "/" + 
                  date.getDate() + "/" + 
                  date.getFullYear();
    return {day: dateKey};
  },
  initial: {value: 0},
  reduce: function(curr, result) { result.value += curr.value; }
});

returns:

[
  {
    "day": "5/4/2011",
    "value": 8
  }
]