Samples, Locations and PickLists / How to avoid bidirectional associations

[UFOMelkor]

Let's start with a short introduction into the domain. We store samples, that (mostly) consist of human body fluid, within large refrigerators. The samples are not stored directly in the refrigerators, because this would make it hard to find them again. Therefore the refrigerators are divided with towers, drawers, shelfs, etc. For this specific problem it is enough to know that the samples are stored into locations that can be moved into other locations.

Next thing existing in the domain are pick lists. Employees search some samples they need an the computer and create a pick list with them. Sometimes later somebody takes the pick list and goes to the different locations to pick the samples. To minimize the distance to walk, the samples are automatically ordered on the pick list by their distance to each other.

There are some situations where IMHO a bidirectional association is necessary between two aggregates.

Let's have a look at one scenario and one part of my model:

Given a pick list with two samples was produced
When someone moves one sample to another location
Then the pick list should be re-sorted

Command	Aggregate	Event
MoveSampleToLocation	Sample.moveToLocation()	SampleWasMovedToLocation
NotifySampleAboutPickList	Sample.notifyAboutPickList()	SampleWasNotifiedAboutPickList
PickSample	Sample.pick()	SampleWasPicked
ProducePickList	PickList::produce()	PickListWasProduced
ResortPickList	PickList.resort()	PickListWasResorted
TickOffSample	PickList.tickOff()	SampleWasTickedOffFromPickList
NotifyAboutSamplePickedOnOtherList	PickList.tryToSubstitute()	SampleWasSubstituted/SampleWasNotSubstituted

ProcessManagers:

Event	Command Target	Command
SampleWasMovedToLocation	∀ lists	ResortPickList
SampleWasPicked	picking list	TickOffSample
SampleWasPicked	∀ other lists	NotifyAboutSamplePickedOnOtherList
PickListWasProduced	∀ samples	NotifySampleAboutPicktList
SampleWasSubstituted	sample	NotifySampleAboutPIckList

The thing I'm worried about: Samples knowing about the pick list that contain them. It does not sound like something, a sample should know. But I don't know how to dispatch commands for all concerned pick lists otherwise. The only thing that would come to my mind are stateful process managers …

[codeliner]

I agree that NotifySampleAboutPickList sounds like a technical command that is not really part of the domain language.

What about using a read model instead of stateful process managers?

$pickLists = $pickListFinder->findBySample($sampleWasMovedToLocation->sampleId())