db={"task":[{"_id":77,"title":"Test title","status":"in-progress","reporter":{"$ref":"User","$id":ObjectId("5daf022549a36e319879f357"),"$db":"db"},"priority":"high","project":{"$ref":"Project","$id":30,"$db":"db"}}],"user":[{"_id":ObjectId("5daf022549a36e319879f357"),"username":"user1","email":"test@gmail.com","is_active":true,"firstName":"user-1"}],"Project":[{"_id":30}]}

db.task.aggregate([{"$lookup":{"from":"user","localField":"reporter.$id","foreignField":"_id","as":"reporter"}},{"$unwind":"$reporter"}])