Database
db={"user":[{"_id":ObjectId("584aac38686860d502929b8b"),"name":"John"}],"role":[{"_id":ObjectId("584aaca6686860d502929b8d"),"role":"Admin","userId":"584aac38686860d502929b8b"}]}
Query
db.user.aggregate([{"$project":{"_id":{"$toString":"$_id"}}},{"$lookup":{"from":"role","localField":"_id","foreignField":"userId","as":"role"}}])