group by day via map reduce

gistfile1.js

var m = function() { 
  	  emit(this._id - (this._id % 86400000), { aggregate:0, count:1, sum: this['threadCount'] } );
    };

var r = function (name, values) {
      var result = { aggregate:0, count:0, sum:0 };
      values.forEach(function(f) {
        result.sum += f.sum;
        result.count += f.count;
      });
      return result;
    };

var f = function (who, res) {
      if (res.count > 0) res.aggregate = res.sum / res.count;
      return res;
    };

db.prod.mapReduce(m, r, {finalize: f, query: { "_id": { $gte: someTimestamp }}, out: { inline : 1 }});