[{"_id":1,"item":"ABC","price":80,"sizes":[{"id":1},{"id":2},{"id":3}]},{"_id":2,"item":"EFG","price":120,"sizes":[{"id":4},{"id":5},{"id":6}]}]

db.collection.aggregate([{"$match":{"_id":1}},{"$match":{"sizes.id":{"$nin":[7]}}},{"$set":{"price":20}}])