Hi Stefan,
This was harder than expected because you can't check associations for empty, which I didn't expect.
I got it to work, but I do not think it's best practice. Since I changed the ownership of the association to both in the domain model. Otherwise, I wasn't able to check if they had a relationship as submitter. Also I think it's weird to add this exercise while a microflow comes to mind first as to tackle this instead of a XPath.
So to accomplish it I made a page with a microflow as data source. Which looks like this:.
First I retrieve all accounts, then I create a new list to store the filtered objects. In the Loop I check if the relationship as submitter is empty. Like this: $IteratorAccount/VacationManagement.VacationRequest_Submitter=empty
If this is false continue the loop otherwise add it to the list and then continue. In the end-event I pass the new filtered list of accounts to show.
Hopefully I explained everything well enough for you to understand. Also I would like to add that these additional challenges are not at all required to get certified as Intermediate. Also there is a discount offer right now that lets you get certified for free: https://www.mendix.com/levelup/