db={"tasklists":[{"_id":1,"tasklist":[1,2,3,4]},{"_id":2,"tasklist":[5,6,7]}],"tasks":[{"_id":1},{"_id":2},{"_id":3},{"_id":5}]}

db.tasklists.aggregate([{"$unwind":{"path":"$tasklist"}},{"$project":{"_id":0,"taskid":"$tasklist"}},{"$lookup":{"from":"tasks","localField":"taskid","foreignField":"_id","as":"joined"}},{"$match":{"$expr":{"$eq":[{"$size":"$joined"},0]}}},{"$unset":["joined"]}])