Aggregation function for mongodb

Please follow this link:

http://www.mongodb.org/display/DOCS/Aggregation

Group by function with aggregate function in mongodb

db.test.group(
   { cond: {"invoked_at.d": {$gte: "2009-11", $lt: "2009-12"}}
   , key: {http_action: true}
   , initial: {count: 0, total_time:0}
   , reduce: function(doc, out){ out.count++; out.total_time+=doc.response_time }
   , finalize: function(out){ out.avg_time = out.total_time / out.count }
   } );

[
  {
    "http_action" : "GET /display/DOCS/Aggregation",
    "count" : 1,
    "total_time" : 0.05,
    "avg_time" : 0.05
  }
]