MongoDB: UpdateOne Record

Question

I've written the following program to update a set records by removing the lowest grade. Based on the output from the program, it appears that the UpdateResult.matchedCount=1 but the UpdateResult.ModifiedCount=0. Essentially, the query is finding the record to be updated, but is not updating the record.

public static void main(String[] args) {

        MongoClient client = new MongoClient();
        MongoDatabase database = client.getDatabase("school");
        MongoCollection<Document> collection = database.getCollection("students");

        FindIterable<Document> iterable = collection.find(new Document("scores.type", "homework")).sort(Sorts.orderBy(Sorts.ascending("_id")));

        MongoCursor<Document> iterableDocument = iterable.iterator();

        while (iterableDocument.hasNext()) {
            Document wholeDocument = (Document) iterableDocument.next();
            List<Document> scores = (List<Document>) wholeDocument.get("scores");

            System.out.println("Student id: " + wholeDocument.get("_id"));
            System.out.println("Student name: " + wholeDocument.get("name"));

            for (final Document score1 : scores) {
                for (final Document score2 : scores) {
                    if (score1.get("type").toString().equalsIgnoreCase("homework") && score2.get("type").toString().equalsIgnoreCase("homework")) {

                        Double homeworkScore1 = (Double) score1.get("score");
                        Double homeworkScore2 = (Double) score2.get("score");

                        if (homeworkScore1 < homeworkScore2) {
                            BasicDBList homeworkScores = new BasicDBList();
                            homeworkScores.add(new BasicDBObject("type", "homework").append("score", homeworkScore1));
                            BasicDBObject lowScoreToRemove = new BasicDBObject("scores", homeworkScores);

                            System.out.println("Lowest Homework score is: " + homeworkScore1);
                            System.out.println("Document that will be updated: " + wholeDocument);
                            System.out.println("Pull Document: " + new Document("$pull", lowScoreToRemove));
                            UpdateResult result = collection.updateOne(new Document("_id", wholeDocument.get("_id")), new BasicDBObject("$pull",
                                    lowScoreToRemove));
                            System.out.println("UpdateResult: " + result + "\n");
                        }
                    }
                }
            }
        }
        iterableDocument.close();
    }

Here is a sample of the output:

Student id: 199
Student name: Rae Kohout
Lowest Homework score is: 5.861613903793295
Document that will be updated: Document{{_id=199, name=Rae Kohout, scores=[Document{{type=exam, score=82.11742562118049}}, Document{{type=quiz, score=49.61295450928224}}, Document{{type=homework, score=28.86823689842918}}, Document{{type=homework, score=5.861613903793295}}]}}
Pull Document: Document{{$pull={ "scores" : [ { "type" : "homework" , "score" : 5.861613903793295}]}}}
UpdateResult: AcknowledgedUpdateResult{matchedCount=1, modifiedCount=0, upsertedId=null}

Answer 1

See line of code where BasicDBList is commented out.

public static void main(String[] args) {

        MongoClient client = new MongoClient();
        MongoDatabase database = client.getDatabase("school");
        MongoCollection<Document> collection = database.getCollection("students");

        FindIterable<Document> iterable = collection.find(new Document("scores.type", "homework")).sort(Sorts.orderBy(Sorts.ascending("_id")));

        MongoCursor<Document> iterableDocument = iterable.iterator();

        while (iterableDocument.hasNext()) {
            Document wholeDocument = (Document) iterableDocument.next();
            List<Document> scores = (List<Document>) wholeDocument.get("scores");

            System.out.println("Student id: " + wholeDocument.get("_id"));
            System.out.println("Student name: " + wholeDocument.get("name"));

            for (final Document score1 : scores) {
                for (final Document score2 : scores) {
                    if (score1.get("type").toString().equalsIgnoreCase("homework") && score2.get("type").toString().equalsIgnoreCase("homework")) {

                        Double homeworkScore1 = (Double) score1.get("score");
                        Double homeworkScore2 = (Double) score2.get("score");

                        if (homeworkScore1 < homeworkScore2) {
                            //BasicDBList homeworkScores = new BasicDBList();
                            //homeworkScores.add(new BasicDBObject("type", "homework").append("score", homeworkScore1));
                            BasicDBObject lowScoreToRemove = new BasicDBObject("scores", new BasicDBObject("type", "homework").append("score", homeworkScore1));

                            System.out.println("Lowest Homework score is: " + homeworkScore1);
                            System.out.println("Document that will be updated: " + wholeDocument);
                            System.out.println("Pull Document: " + new Document("$pull", lowScoreToRemove));
                            UpdateResult result = collection.updateOne(new Document("_id", wholeDocument.get("_id")), new BasicDBObject("$pull",
                                    lowScoreToRemove));
                            //iterable.
                            System.out.println("UpdateResult: " + result + "\n");
                        }
                    }
                }
            }
        }
        iterableDocument.close();
    }

Answer 2

My mongo version doesn't have updateOne, but at least from my experience with 'update' the parameter should not be an array.

So instead of:
{$pull:{scores:[{type:'homework', score:5.5...}]}}); 
it should be (note the lack of "["):
{$pull:{scores:{type:'homework', score:5.5...}}}); 

Regardless, note there are reported bugs/unintuitive features about WriteResult, so I suggest to also manually look at your collection to see what's been deleted.