db={"user":[{"_id":1,"name":"user 1"},{"_id":2,"name":"user 2"},{"_id":3,"name":"user 3"}],"transaction":[{"_id":1,"user":1},{"_id":2,"user":3}]}

db.user.aggregate([{"$lookup":{"from":"transaction","localField":"_id","foreignField":"user","as":"trans"}},{"$match":{"trans.user":{"$exists":false}}}])