Group by subdocument field using aggregation framework - mongodb

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