MongoDB C＃并发问题与upsert

Question

I have a quite simple method for upserting an entry in a collection:

    public void UpsertEntry(string collectionName, Entry entry)
    {
        var collection = GetCollection(collectionName);
        var filter = Builders<Entry>.Filter.Eq(x => x.Key, entry.Key);
        var update = Builders<Entry>.Update
            .SetOnInsert(x => x.Id, ObjectId.GenerateNewId())
            .SetOnInsert(x => x.Key, entry.Key)
            .Set(x => x.LastUpdated, DateTime.Now)
            .Set(x => x.Data, entry.Data)
            .Inc(x => x.Version, 1);

        var result = collection.UpdateOne(filter, update, new UpdateOptions
        {
            IsUpsert = true
        });
    }

The collection has a unique index on Key.

I'm calling the UpsertEntry method in a Parallel.For loop

        Parallel.For(0, 100, i =>
        {
            entry.Key = $"key+{i}";
            UpsertEntry(coll, entry);
        });

I'd expect to get 100 record inserted, but instead I receive a E11000 duplicate key error collection: TestDB.test::c7 index: key_1 dup key: { : "key+1" } . Why am I experiencing concurrency issues? What's wrong with my code?

Answer 1

The error message is saying that there's already a record with null. In other words, it means that you already have one with the same key represented in the collection

The relevant documentation for this:

If a document does not have a value for the indexed field in a unique index, the index will store a null value for this document. Because of the unique constraint, MongoDB will only permit one document that lacks the indexed field. If there is more than one document without a value for the indexed field or is missing the indexed field, the index build will fail with a duplicate key error.

You can combine the unique constraint with the sparse index to filter these null values from the unique index and avoid the error.

unique indexes

Sparse indexes only contain entries for documents that have the indexed field, even if

the index field contains a null value.

In other words, a sparse index is ok with multiple documents all having null values.

Answer 2

I resolved it and it has nothing to do with the driver itself. I didn't noticed I was re-using the same instance of the class Entry :-)