Group by subdocument field using aggregation framework

Question

The structure is the following:

{
    "_id" : "79f00e2f-5ff6-42e9-a341-3d50410168de",
    "bookings" : [
        {
            "name" : "name1",
            "email" : "george_bush@gov.us",
            "startDate" : ISODate("2013-12-31T22:00:00Z"),
            "endDate" : ISODate("2014-01-09T22:00:00Z")
        },
        {
            "name" : "name2",
            "email" : "george_bush@gov.us",
            "startDate" : ISODate("2014-01-19T22:00:00Z"),
            "endDate" : ISODate("2014-01-24T22:00:00Z")
        }
    ],
    "name" : "Hotel0",
    "price" : 0,
    "rating" : 2 
}

Now, I want to generate a report telling me how many bookings were made, grouped by booking month (assume that only booking start date matters) and also grouped by hotels rating.

I expect the answer to be like that:

{
    {
        rating: 0,
        counts: {
            month1: 10,
            month2: 20,
            ...
            month12: 7
        }
    }
    {
        rating: 1,
        counts: {
            month1: 5,
            month2: 8,
            ...
            month12: 9
        }
    }
    ...
    {
        rating: 6,
        counts: {
            month1: 22,
            month2: 23,
            ...
            month12: 24
        }
    }
}

I tried this with aggregation framework but I'm a little bit stuck.

Answer 1

The following query:

db.book.aggregate([
    { $unwind: '$bookings' },
    { $project: { bookings: 1, rating: 1, month: { $month: '$bookings.startDate' } } },
    { $group: { _id: { rating: '$rating', month: '$month' }, count: { $sum: 1 } } }
]);

Will give you the result per rating/month, but it does not make a subdocument for months. In general, you can not convert a value (such as the month nr) to a key (such as month1)—this is something you can probably quite easily handle in your application though.

The above aggregation results in:

"result" : [
    {
        "_id" : {
            "rating" : 2,
            "month" : 1
        },
        "count" : 1
    },
    {
        "_id" : {
            "rating" : 2,
            "month" : 12
        },
        "count" : 1
    }
],
"ok" : 1