mongodb .net driver - Unknown discriminator value 'MyEvent'

Question

Using the MongoDB persistance engine in joliver/EventStore causing the error Unknown discriminator value 'MyEvent'. The issue is only caused when I try to load all events for replaying the events like this.storeEvent.Advanced.GetFrom(new DateTime(2010, 1,1))

The issues is caused in ExtensionsMethods.cs

public class MyClassEvent : IDomainEvent { ... }

public static Commit ToCommit(this BsonDocument doc, IDocumentSerializer serializer)
    {
        if (doc == null)
            return null;

        var id = doc["_id"].AsBsonDocument;
        var streamId = id["StreamId"].AsGuid;
        var commitSequence = id["CommitSequence"].AsInt32;

        var events = doc["Events"].AsBsonArray.Select(e => e.AsBsonDocument["Payload"].IsBsonDocument ? BsonSerializer.Deserialize<EventMessage>(e.AsBsonDocument["Payload"].AsBsonDocument) : serializer.Deserialize<EventMessage>(e.AsBsonDocument["Payload"].AsByteArray)).ToList();
        var streamRevision = doc["Events"].AsBsonArray.Last().AsBsonDocument["StreamRevision"].AsInt32;
        return new Commit(
            streamId,
            streamRevision,
            doc["CommitId"].AsGuid,
            commitSequence,
            doc["CommitStamp"].AsDateTime,
            BsonSerializer.Deserialize<Dictionary<string, object>>(doc["Headers"].AsBsonDocument),
            events);
    }

My configuration is like this:

 Wireup.Init()                
            .UsingMongoPersistence(connectionName, new DocumentObjectSerializer())
            .UsingBsonSerialization()    
            .UsingAsynchronousDispatcher()                                
            .PublishTo(this.container.Resolve<IPublishMessages>())
            .Build();

But have tried almost all kind of serializer options.

Answer 1

I just ran into this too. Zsolt's answer was a good starting point, but I ended up solving it slightly different.

Note that I did not only get this when myEventStore.Advanced.GetFrom(...); myEventStore.OpenStream(...) also fails. This makes sense, because both methods use the same IPersistentStream and serializer.

I don't run into this problem when when I first persist an event, before retrieving an event of the same type. Apparently MongoDB creates a ClassMap when it's being asked to serialize a type for the first time.

Anyway, for me the solution was to create a class map for all my event types on application startup. Assuming all type are in the assembly of SimpleCQRS.Event and derive from SimpleCQRS.Event, I do it like this:

var types = Assembly.GetAssembly(typeof(SimpleCQRS.Event))
                    .GetTypes()
                    .Where(type => type.IsSubclassOf(typeof(SimpleCQRS.Event)));
foreach (var t in types)
    BsonClassMap.LookupClassMap(t);

For me this works better than using BsonClassMap.RegisterClassMap<TypeToMap> like Zsolt suggests, because that requires a generic type parameter, meaning you have to manually add each event type.