LoadAsync immediately after SaveChangesAsync returns null

[thomaslevesque]

Hi there,
Probably a noob question, I have no experience with Marten or with Event Sourcing, just experimenting at the moment.
I have a TodoProjection registered as Inline, and an API endpoint like this:

app.MapPost("todo/new", async (CreateTodoRequest request, IDocumentSession session) =>
{
    var id = Guid.NewGuid();
    var @event = new TodoCreated(id, request.Text, request.DueDate);
    session.Events.StartStream<Todo>(@event);
    await session.SaveChangesAsync();
    return await session.LoadAsync<Todo>(id);
}).WithName("Create Todo");

The Todo is correctly created (I can get it in a subsequent request), but LoadAsync always returns null.

What am I missing?

[thomaslevesque]

I also tried to change session.LoadAsync to session.Events.AggregateStreamAsync, but it also returns null.
How can I get the initial state of the aggregate right after starting the event stream?

[mysticmind]

Could you please share your full code to check?

[thomaslevesque]

Hi @mysticmind,
Here's my full code: https://gist.github.com/thomaslevesque/cb454b1fdc3cd2661af54ea072ae5f1a
The issue is on line 41 of Program.cs

[mysticmind]

@thomaslevesque I figured out the issue, change session.Events.StartStream<Todo>(@event); to session.Events.StartStream<Todo>(id, @event);. Essentially, we are setting the stream Id same as the Todo Id otherwise the stream will generate it's own Id which will reflect as the Todo Id in the projected document.

[thomaslevesque]

Works perfectly now, thanks!
TBH it's a bit surprising. Since Marten can obviously figure out that Todo.Id is the stream id (since I can get the projected document later using this id), why doesn't it do it automatically?
Anyway, this fixes the problem. Thanks again!

[mysticmind]

Stream Id will be the Todo Id in the doc. If you closely look up the event entries with the problem you had, the Todo Id which you passed via var id = Guid.NewGuid(); and the Stream Id set would differ in this case. If you look up the Todo record Id in db and try to fetch it based on what you see in the db entry, you will get the data. But the Id is not the same as what you originally wanted to use (it is an auto generated stream id since you didn't pass one while creating the stream).

[mysticmind]

You can double check this, check the Id in your event entries, stream and Todo doc. All have to match for the the implementation which you are attempting to do.

Without the fix for StartStream(...):

• Stream and Todo doc Id will match
• Id in event entries (which is your intended Id) won't match with the both Stream/Todo Id's.

[thomaslevesque]

I see, thanks for the explanation!