db={"ask":[{"_id":1,guess:1},{"_id":2,guess:2}],"guess":[{"_id":1,"body":"findme"},{"_id":2,"body":"test"}]}

db.ask.aggregate({$unwind:"$guess"},{$lookup:{from:"guess",localField:"guess",foreignField:"_id",as:"guess"}},{$match:{"guess.body":"test"}})