Question:

I have a collection

{ "_id" : 1325376000, "value" : 13393} { "_id" : 1325462400, "value" : 13393}

ObjectIds are Unix Timestamp and are storing as Number manually.(at insert time).

now I'm searching for a solution that i could calculate sum of values for each month with Aggregation Framework.

Here is a way you can do it by generating the aggregation pipeline programmatically:

numberOfMonths=24; /* number of months you want to go back from today's */ now=new Date(); year=now.getFullYear(); mo=now.getMonth(); months=[]; for (i=0;i<numberOfMonths;i++) { m1=mo-i+1; m2=m1-1; d = new Date(year,m1,1); d2=new Date(year,m2,1); from= d2.getTime()/1000; to= d.getTime()/1000; dt={from:from, to:to, month:d2}; months.push(dt); } prev="$nothing"; cond={}; months.forEach(function(m) { cond={$cond: [{$and :[ {$gte:["$_id",m.from]}, {$lt:["$_id",m.to]} ]}, m.month, prev]}; prev=cond; } ); /* now you can use "cond" variable in your pipeline to generate month */ db.collection.aggregate( { $project: { month: cond , value:1 } }, { $group: {_id:"$month", sum:{$sum:"$value"} } } )