Question

Assuming Stack Overflow domain problem and the following definition of events:

UserRegistered(UserId, Name, Email)
UserNameChanged(UserId, Name)
QuestionAsked(UserId, QuestionId, Title, Question)

Assuming the following state of event store (in the order of appearance):

1) UserRegistered(1, "John", "[e-mail]")
2) UserNameChanged(1, "SuperJohn")
3) UserNameChanged(1, "John007")
4) QuestionAsked(1, 1, "Help!", "Please!")

Assuming the following denormalized read model for questions listing (for the first page of SO):

QuestionItem(UserId, QuestionId, QuestionTitle, Question, UserName)

And the following event handler (which builds denormalized read model):

public class QuestionEventsHandler
{
    public void Handle(QuestionAsked question)
    {
        var item = new QuestionItem(
            question.UserId, 
            question.QuestionId, 
            question.Title, 
            question.Question, 
            ??? /* how should i get name of the user? */);
        ...
    }
}

My question is how can i find the name of the user who asked a question? Or more common: how should i handle events if my denormalized read model requires additional data which is not exists in the particular event?

I’ve examined existing samples of CQRS including SimpleSQRS of Greg Young and Fohjin sample of Mark Nijhof. But it seems to me that they are operate only with data that is included in events.

Answer 1

Just enrich the event with all the necessary information.

Greg’s approach, as I recall, – enrich the event while creating and store/publish it this way.