notify_missing_spool_assignments_on_print_start queried only the legacy
SpoolAssignment table. In Spoolman mode that table is empty -- bindings
live in spoolman_slot_assignments -- so assigned_global_trays came back
empty and every used tray was flagged missing, firing a false-positive
notification on every print.
Union SpoolAssignment + SpoolmanSlotAssignment rows for the printer
before computing the missing set. Both tables expose printer_id /
ams_id / tray_id identically, so _global_tray_from_assignment is
unchanged. Union-only, so legacy-mode behavior cannot regress.
maziggy
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