cqrs - How to "join" two Aggregate Roots when preparing View Model? -

assume book , author aggregate roots in model.

in read model have table authorsandbooks list of authors , books joined book.authorid

when bookadded event fired want receive author data create new authorsandbooks line.

because book aggregate root, information author doesn't included in bookadded event. , cannot include because author root doesn't have getters (according guidelines of examples , posts cqrs , event sourcing).

usually receive 2 types of answers on question:

1. enrich domain event data need in event handlers. said cannot aggregates roots.
2. use available data view model. i.e. load author view model , use build authorsandbooks row.

the last 1 has problems concurrency. author data can not available in view model @ time bookadded event handling.

what approach use solve this? thank you.

as general advice, let event handlers idempotent , make sure can deal out of order message handling (either re-queuing or building in mechanisms fill in missing data). on other hand, question why author , book such desperate aggregate roots. maybe should copy author upon adding book (what f* "adding book", how's command). problem these made-up examples. descend real world, doubt problem exists.