Can I invoke a Process Builder process on all Child Objects?

Question

I'm trying to invoke a process builder process on all Contacts related to an Account, when an Activity (Event) related to that Account is updated.

I know I can update the full set of contact records themselves, for all child Contacts of the Account, using the Update Records action:

Action Type: Update Records

Record Type: [Event].Account ID.Contacts <--this represents all Contacts of the related Account

...but can I pass all the Account's Contacts as the Starting Object(s) for an invoked process? It seems I can only pass a single reference as the Starting Object:

When trying to define the reference for the SObject variable, I navigate a similar UI as when I selected the "Record Type" (record(s) to update) for the Update Records Action, but I can only select parent objects of [Event].Account ID, and [Event].Account ID.Contacts isn't selectable.

I realize I could create a custom "Run Update" field on the Contact object, and update this flag using the Update Records action from the Event-triggered process, then have this Run Update field trigger a process on each Contact record individually, but this seems sloppy. Is there any way to invoke the process on a set of objects like this, without this weird workaround and without Apex?

Answer 1

I think you can just pass the Event as sObject reference and your Update Records process will take care of updates of all contacts, since Record Type is [Event].Account ID.Contacts.