Sort by date in mongoose aggregation framework

Question

Im working on a nodejs+mongodb project using mongoose. Now I have come across a question I don't know the answer to. I am using aggregation framework to get grouped results. The grouping is done on a date excluding time data field like: "2013 02 06". Code looks like this:

MyModel.aggregate([
            {$match: {$and: [{created_date: {$gte: start_date}}, {created_date: {$lte: end_date}}]}},
            {$group: {
                _id: {
                    year: {$year: "$created_at"},
                    month: {$month: "$created_at"},
                    day: {$dayOfMonth: "$created_at"}
                },
                count: {$sum: 1}
            }},
            {$project: {
                date: {
                        year: "$_id.year",
                        month:"$_id.month",
                        day:"$_id.day"
                },
                count: 1,
                _id: 0
            }}
        ], callback);

The grouped results are perfect, except that they are not sorted. Here is an example of output:

[
    {
        count: 1,
        date: {
            year: 2013,
            month: 2,
            day: 7
        }
    },
    {
        count: 1906,
        date: {
            year: 2013,
            month: 2,
            day: 4
        }
    },
    {
        count: 1580,
        date: {
            year: 2013,
            month: 2,
            day: 5
        }
    },
    {
        count: 640,
        date: {
            year: 2013,
            month: 2,
            day: 6
        }
    }
]

I know the sorting is done by adding this: {$sort: val}. But now I'm not sure what should be the val so the results would be sorted by date as my grouping key es an object of 3 values constructing the date. Does anyone know how this could be accomplished?

EDIT Have tryed this and it worked :)

{$sort: {"date.year":1, "date.month":1, "date.day":1}}

Answer 1

It appears that this question has a very simple answer :) Just need to sort by multiple nesteed columns like this:

{$sort: {"date.year":1, "date.month":1, "date.day":1}}