MongoDB 的唯一 int _id

Question

I am trying to assign unique int for _id. I know that ObjectID is a great option, but I need to use int as an ID in my case. I am using Random to generate random int to assign it the new object.

Here is the error:

Exception in thread "main" com.mongodb.MongoWriteException: E11000 duplicate key error 
collection: TOP100SongsUK.artists index: _id_ dup key: { _id: 484624875 }

Though, it is not true. For now, my collection has a few records and that's impossible. What may be a trigger this issue?

Here is the method that writes into the DB and last method assigns an ID.

 public boolean add(Artist artist) {
    Document artistToInsert = new Document();
    artistToInsert.append(ARTIST_ID, assignArtistID()).append(ARTIST_NAME, artist.getName());
    boolean validOperation = false;
    int attempts = 0;
    do {
        try {
            addArtist(artistToInsert);
            validOperation = true;
        } catch (DuplicateKeyException e) {
            attempts++;
            System.out.println("Incorrect ID for Artist");
        }
    } while (!validOperation || attempts < 3);
    if (attempts == 3) {
        throw new RuntimeException("Unable to add Artist - Mongo");
    }
    return true;
}

private boolean addArtist(Document artistToInsert) {
    artistCollection.insertOne(artistToInsert);
    return true;
}

private static Random intGenerator = new Random();
 private int assignArtistID() {
        return Math.abs(intGenerator.nextInt());
}

UPDATED: getNextSequence Function:

static final String ID_PARAM = "idParam";
static final String ID_SEQ = "seq";

static int getNextSequence(MongoCollection<Document> collection){
    Document findRequest = new Document();
    findRequest.append("_id", ID_PARAM);
    Document updateRequest =  new Document();
    updateRequest.append("$inc", new Document(ID_SEQ,1));
    FindOneAndUpdateOptions options = new FindOneAndUpdateOptions();
    options.upsert(true);
    options.returnDocument(ReturnDocument.AFTER);
    Document returnDoc =  collection.findOneAndUpdate(findRequest,updateRequest,options);
    return (int) returnDoc.get(ID_SEQ);
}

Answer 1

It's not a good idea to use a random int to generate your _id mongo. You should use an auto increment sequence using Mongo.

You have to create a collection for your sequence, with a sequenceName and your sequenceNumber. Then for each insert, get your sequence, and increment it.

Here are some links to help you with that :

• Auto increment sequence in mongodb using java
• https://mkyong.com/mongodb/spring-data-mongodb-auto-sequence-id-example/
• https://dzone.com/articles/mongodb-java-api-using-a-sequence-collection-with

Answer 2

The issue was with the code itself, not an auto-increment sequence. The loop was going on the second iteration, so the method to insert Object was called twice, but ID was the same in both cases.

The first call wrote to db with correctly incremented _id, but the loop didn't break. Thus the same object with the same id was writing to db and this caused WriteMongoException.

I removed a loop, works perfectly fine