How to model sort order for many-to-one across two aggreagate roots

Take the domain proposed in Effective Aggregate Design of a Product which has multiple Releases. In this article, Vaughn arrives at the conclusion that both the Product and Release should each be their own aggregate roots.

Now suppose that we add a feature

As a release manager I would like to be able to sort releases so that I can create timelines for rolling out larger epics to our users

I'm not a PM with a specific need but it seems reasonable that they would want the ability to sort releases in the UI.

I'm not exactly sure how this should work. Its natural for each Release to have an order property but re-ordering would involve changing multiple aggregates on the same transaction. On the other hand, if that information is stored in the Product aggregate you have to have a method like product.setRelaseOrder(ReleaseId[]) which seems like a weird bit of data to store at a completely different place than Releases. Worse, adding a release would again involve modification on two different aggregates! What else can we do? ProductReleaseSortOrder can be its own aggregate, but that sounds downright absurd!

So what to do? At the moment I'm still leaning toward the let-product-manage-it option but what's correct here?

Solution

So, Product and Release are both ARs. Release has an association to Product via AggregateId. You want to get list of all releasesfor a given product ordered by something?

Since ordering is an attribute of aggregate, then it should be set on Product, but Releases are ARs too and you shouldn't access repository of Release in Product AR (every AR should have its own repository).

I would simply make a ReleaseQueryService that takes productId and order parameter and call ReleaseRepository.loadOrderedReleasesForProduct(productId, order).

I would also think about separating contexts, maybe model for release presentation should be in another context? In example additional AR ProductReleases that would be used only for querying.