Mongoose aggregate issue with grouping by nested field - mongodb

Question

I have a schema that stores user attendance for events:
event:
_id: ObjectId,
...
attendances: [{
user: {
type: ObjectId,
ref: 'User'
},
answer: {
type: String,
enum: ['yes', 'no', 'maybe']
}
}]
}
Sample data:
_id: '533483aecb41af00009a94c3',
attendances: [{
user: '531770ea14d1f0d0ec42ae57',
answer: 'yes',
}, {
user: '53177a2114d1f0d0ec42ae63',
answer: 'maybe',
}],
I would like to return this data in the following format when I query for all attendances for a user:
var attendances = {
yes: ['533497dfcb41af00009a94d8'], // These are the event IDs
no: [],
maybe: ['533497dfcb41af00009a94d6', '533497dfcb41af00009a94d2']
}
I am not sure the aggegation pipeline will return it in this format? So I was thinking I could return this and modify it easily:
var attendances = [
answer: 'yes',
ids: ['533497dfcb41af00009a94d8'],
},{
answer: 'no',
ids: ['533497dfcb41af00009a94d8']
}, {
answer: 'maybe',
ids: ['533497dfcb41af00009a94d6', '533497dfcb41af00009a94d2']
}];
My attempt is not succesful however. It doesn't group it by answer:
this.aggregate({
$match: {
'attendances.user': someUserId
}
}, {
$project: {
answer: '$attendances.answer'
}
}, {
$group: {
_id: '$answer',
ids: {
$addToSet: "$_id"
}
}
}, function(e, docs) {
});
Can I return the data I need in the first desired format and if not how can I correct the above code to achieve the desired result?
On that note - perhaps the map-reduce process would be better suited?

Answer 1

The below query will help you get close to the answer you want. Although, it isn't exactly in the same format you are expecting, you get separate documents for each answer option and an array of event id's.
db.collection.aggregate([
// Unwind the 'attendances' array
{"$unwind" : "$attendances"},
// Match for user
{"$match" : {"attendances.user" : "53177a2114d1f0d0ec42ae63"}},
// Group by answer and push the event id's to an array
{"$group" : {_id : "$attendances.answer", eventids : {$push : "$_id"}}}
])
This produces the below output:
{
"result" : [
{
"_id" : "yes",
"eventids" : [
ObjectId("53d968a8c4840ac54443a9d6")
]
},
{
"_id" : "maybe",
"eventids" : [
ObjectId("53d968a8c4840ac54443a9d7"),
ObjectId("53d968a8c4840ac54443a9d8")
]
}
],
"ok" : 1
}