There are a few places where it is useful to grab the association between two identities, which can greatly simplify the read models and sagas.

For example:
AccountByUserIdReadModel
Handles AccountCreatedEvent

• UserId
• etc.

Handles AccountInfoUpdatedEvent

The way I see it there are three options to solve this. Either:

• In the Locator: Use the AccountId on an existing read model to find the UserId and return that as the ReadModelId
• In the Query Handler: Create a custom query & handler that does this mapping in-app or in the database
• After the read model: Does two calls to the database two build the overall data view

In both the Locator and the Query Handler you can benefit for a specialize look-up that maps relationships. In this case, AccountId to UserId(s).

Throughout the years, my teams have taken various approaches in solving this problem. I would be interested in hearing how others have gone about it.

Might be worth building in a standard into EventFlow