MongoDB aggregation/map-reduce - mongodb

Question

I'm new to MongoDB and I need to do an aggregation which seems to me quite difficult. A document looks something like this
{
"_id" : ObjectId("568192aef8bd6b0cd0f649c6"),
"conference" : "IEEE International Conference on Acoustics, Speech and Signal Processing",
"prism:aggregationType" : "Conference Proceeding",
"children-id" : [
"SCOPUS_ID:84948148564",
"SCOPUS_ID:84927603733",
"SCOPUS_ID:84943521758",
"SCOPUS_ID:84905234683",
"SCOPUS_ID:84876113709"
],
"dc:identifier" : "SCOPUS_ID:84867598678"
}
The example contains just the fields I need in the aggregation. Prism:aggregationType can have 5 different values(conference proceeding, book, journal etc.). Children-id says that this document is cited by an array of other documents(SCOPUS_ID is an unique ID for each document).
What I want to do is to group first by conference, then for each conference I want to know for each prism:aggregationType how many citing documents are($gt > 0).
For example, lets say there are 100 documents that have the conference from above. These 100 documents are cited by 250 documents. I want to know from all of these 250 documents how many have "prism:aggregationType" : "Conference Proceeding", "prism:aggregationType" : "Journal" etc.
An output could look like this:
{
"conference" : "IEEE International Conference on Acoustics, Speech and Signal Processing",
"aggregationTypes" : [{"Conference Proceeding" : 50} , {"Journal" : 200}]
}
It is not important if it is done with aggregation pipeline or map-reduce.
EDIT
Is there any way to combine these 2 into one aggregation:
db.articles.aggregate([
{ $match:{
conference : {$ne : null}
}},
{$unwind:'$children-id'},
{$group: {
_id: {conference: '$conference'},
'cited-by':{$push:{'dc:identifier':"$children-id"}}
}}
]);
db.articles.find( { 'dc:identifier': { $in: [ 'SCOPUS_ID:84943302953', 'SCOPUS_ID:84927603733'] } }, {'prism:aggregationType':1} );
In the query I want to replace the array from $in with the array created with $push

Answer 1

Please try this one through aggregation
> db.collections
.aggregate([
// 1. get the size of `children-id` array through $project
{$project: {
conference: 1,
IEEE1: 1,
'prism:aggregationType': 1,
'children-id': {$size: '$children-id'}
}},
// 2. group by `conference` and `prism:aggregationType` and sum the size of `children-id`
{$group: {
_id: {
conference:'$conference',
aggregationType: '$prism:aggregationType'
},
ids: {$sum: '$children-id'}
}},
// 3. group by `conference`, and make pair of the conference processing ids size and journal ids size
{$group: {
_id: '$_id.conference',
aggregationTypes: {
$cond: [{$eq: ['$_id.aggregationType', 'Conference Proceeding']},
{$push: {"Conference Proceeding": '$ids'}},
{$push: {"Journal": '$ids'}}
]}
}}
]);

Answer 2

As we had a chat,
using $lookup in aggregation pipeline is unfortunately bonded to mongodb 3.2 which is not a case as R driver can use mongo 2.6 and source documents are in more than one collection.

Answer 3

The code I wrote in the EDIT section is also the final result I come up with(a little bit modified)
db.articles.aggregate([
{ $match:{
conference : {$ne : null}
}},
{$unwind:'$children-id'},
{$group: {
_id: '$conference',
'cited-by':{$push:"$children-id"}
}}
]);
db.articles.find( { 'dc:identifier': { $in: [ 'SCOPUS_ID:84943302953', 'SCOPUS_ID:84927603733'] } }, {'prism:aggregationType':1} );
The result will look like this for each conference:
{
"_id" : "Annual Conference on Privacy, Security and Trust",
"cited-by" : [
"SCOPUS_ID:84942789431",
"SCOPUS_ID:84928151617",
"SCOPUS_ID:84939229259",
"SCOPUS_ID:84946407175",
"SCOPUS_ID:84933039513",
"SCOPUS_ID:84942789431",
"SCOPUS_ID:84942607254",
"SCOPUS_ID:84948165954",
"SCOPUS_ID:84926379258",
"SCOPUS_ID:84946771354",
"SCOPUS_ID:84944223683",
"SCOPUS_ID:84942789431",
"SCOPUS_ID:84939169499",
"SCOPUS_ID:84947104346",
"SCOPUS_ID:84948764343",
"SCOPUS_ID:84938075139",
"SCOPUS_ID:84946196118",
"SCOPUS_ID:84930820238",
"SCOPUS_ID:84947785321",
"SCOPUS_ID:84933496680",
"SCOPUS_ID:84942789431"
]
}
I iterate through all the documents I get (around 250) and then I use the cited-by array inside $in. I use index over dc:identifier so it works instantly.
$lookup could be an alternative to get the things done from aggregate pipeline but packages in R does not support versions above 2.6.
Thank you for your time anyway :)