db={"groups":[{"id":1,"start":10.12,"linkedUsers":[ObjectId("5bfd2fb351503b75fa401817"),ObjectId("5bfd35f051503b75fa40181a")]}],"users":[{"_id":ObjectId("5bfd2fb351503b75fa401817"),"name":"Hardik"},{"_id":ObjectId("5bfd35f051503b75fa40181a"),"name":"Kartik"}],"photos":[{"_id":ObjectId("5bfd329451503b75fa401819"),"userId":ObjectId("5bfd2fb351503b75fa401817"),"location":""},{"_id":ObjectId("5bfd360a51503b75fa40181b"),"userId":ObjectId("5bfd35f051503b75fa40181a"),"location":"AHMDB"}],"tags":[{"_id":ObjectId("5bfd327251503b75fa401818"),"userId":ObjectId("5bfd2fb351503b75fa401817"),"rounds":3,"time":180},{"_id":ObjectId("5bfd361551503b75fa40181c"),"userId":ObjectId("5bfd35f051503b75fa40181a"),"rounds":3,"time":180}]}

db.groups.aggregate([{$unwind:"$linkedUsers"},{$lookup:{from:"users",localField:"linkedUsers",foreignField:"_id",as:"users"}},{$unwind:"$users"},{$lookup:{from:"photos",localField:"linkedUsers",foreignField:"userId",as:"photos"}},{$unwind:"$photos"},{$lookup:{from:"tags",localField:"linkedUsers",foreignField:"userId",as:"tags"}},{$unwind:"$tags"},{$group:{_id:"$_id","start":{$first:"$start"},"linkedUsers":{$first:"$linkedUsers"},"users":{"$push":{"id":"$users._id","name":"$users.name","rounds":"$tags.rounds","time":"$tags.time","photos":"$photos"}}}}])