MongoDB .NET not generating _id on upsert

Question

I\'m attempting to upsert a document into MongoDB 2.4.4 using the .NET driver. It seems not to automatically generate the _id on upserts, though it does correct

Answer 1

And of course I find the answer immediately after posting the question. From this answer, the solution is to add a [BsonIgnoreIfDefault] attribute to the ID. In the example from the question it would be:

public class MongoObject
{
    [BsonId(IdGenerator = typeof(StringObjectIdGenerator))]
    [BsonRepresentation(BsonType.ObjectId)]
    [BsonIgnoreIfDefault]    // <--- this is what was missing
    public string MongoID { get; set; }

    public int Index { get; set; }
}