This solution won't work if the full form is re-evaluated because the result of the join will include all values saved and the validation will fail for every question. Enketo seems to sometimes do a full re-evaluation but not always (this is something we intend to harmonize between Collect and Enketo at some point). Collect always does the full re-evaluation so the form will not work.
I believe this form works in all cases across Collect and Enketo.