db={"questions":[{"_id":1,"title":"first"}],"answers":[{"_id":1,"questionId":1,"content":"test"},{"_id":2,"questionId":1,"content":"test"}],"comments":[{"_id":1,"idQuestion":1,"idAnswer":1,"content":"hello"},{"_id":2,"idQuestion":1,"idAnswer":2,"content":"hi"}]}

db.answers.aggregate([{$match:{questionId:1}},{$lookup:{from:"comments",localField:"_id",foreignField:"idAnswer",as:"comments"}}])