Database
db={"users":[{"_id":"user1","name":"John"},{"_id":"user2","name":"Jane"},{"_id":"user3","name":"Mike"},{"_id":"user4","name":"Anna"},{"_id":"user5","name":"Tom"}],"files":[{"_id":"fileID1","url":"url1","userID":"user1"},{"_id":"fileID2","url":"url2","userID":"user1"},{"_id":"fileID3","url":"url3","userID":"user1"},{"_id":"fileID4","url":"url4","userID":"user1"},{"_id":"fileID7","url":"url7","userID":"user2"},{"_id":"fileID8","url":"url8","userID":"user2"},{"_id":"fileID9","url":"url9","userID":"user2"},{"_id":"fileID10","url":"url10","userID":"user2"},{"_id":"fileID11","url":"url11","userID":"user2"},{"_id":"fileID12","url":"url12","userID":"user2"},{"_id":"fileID13","url":"url13","userID":"user3"},{"_id":"fileID14","url":"url14","userID":"user3"},{"_id":"fileID15","url":"url15","userID":"user3"},{"_id":"fileID16","url":"url16","userID":"user3"},{"_id":"fileID17","url":"url17","userID":"user3"},{"_id":"fileID19","url":"url19","userID":"user4"},{"_id":"fileID20","url":"url20","userID":"user4"},{"_id":"fileID21","url":"url21","userID":"user5"},{"_id":"fileID22","url":"url22","userID":"user5"},{"_id":"fileID23","url":"url23","userID":"user5"},{"_id":"fileID24","url":"url24","userID":"user5"},{"_id":"fileID25","url":"url25","userID":"user5"},{"_id":"fileID26","url":"url26","userID":"user5"},{"_id":"fileID27","url":"url27","userID":"user5"}]}
Query
db.files.aggregate([{$match:{userID:{$in:["user1","user2","user5"]}}},{$group:{_id:"$userID",v:{$push:"$_id"}}},{$project:{v:{$slice:["$v",5]}}}])