-- Auto-charge tuition when a student is marked present or late.
--
-- Idempotency is the whole problem here. The attendance UI upserts on
-- (student_id, date) and teachers freely toggle a status — present, then absent,
-- then present again. A naive "insert a charge on attendance" trigger bills the
-- family once per click. So each auto-charge is bound to the attendance row that
-- caused it via ledger_entries.attendance_id (UNIQUE), which makes the write an
-- upsert and lets a status change back to absent *remove* the charge.
--
-- Manual ledger entries keep attendance_id NULL. Postgres allows unlimited NULLs
-- in a unique index, so hand-entered charges are unaffected, and the UI can tell
-- auto from manual by whether attendance_id is set.

-- Per-student daily rate. NULL (the default) means "never auto-charge", so
-- nothing starts billing until a rate is deliberately set on a student.
ALTER TABLE public.students
  ADD COLUMN IF NOT EXISTS daily_tuition_cents INTEGER;

ALTER TABLE public.ledger_entries
  ADD COLUMN IF NOT EXISTS attendance_id UUID REFERENCES public.attendance(id) ON DELETE CASCADE;

-- One auto-charge per attendance record, and deleting the attendance record
-- takes its charge with it (ON DELETE CASCADE above).
DO $$ BEGIN
  ALTER TABLE public.ledger_entries ADD CONSTRAINT ledger_entries_attendance_id_key UNIQUE (attendance_id);
EXCEPTION WHEN duplicate_table OR duplicate_object THEN NULL; END $$;

-- SECURITY DEFINER because the writer is a *teacher* marking attendance, and
-- ledger_entries is admin-write under RLS. The teacher never gains general
-- ledger access — only this function's fixed, attendance-derived write.
CREATE OR REPLACE FUNCTION public.sync_attendance_tuition()
RETURNS TRIGGER LANGUAGE plpgsql SECURITY DEFINER SET search_path = public AS $$
DECLARE
  rate INTEGER;
BEGIN
  SELECT daily_tuition_cents INTO rate FROM public.students WHERE id = NEW.student_id;

  -- 'present' and 'late' are billable: the student attended either way.
  -- 'absent' and 'excused' are not.
  IF NEW.status IN ('present', 'late') AND rate IS NOT NULL AND rate > 0 THEN
    INSERT INTO public.ledger_entries
      (student_id, date, kind, category, amount_cents, note, created_by, attendance_id)
    VALUES
      (NEW.student_id, NEW.date, 'charge', 'tuition', rate,
       'Auto-charged from attendance (' || NEW.status || ')', NEW.recorded_by, NEW.id)
    ON CONFLICT (attendance_id) DO UPDATE
      SET amount_cents = EXCLUDED.amount_cents,
          date         = EXCLUDED.date,
          note         = EXCLUDED.note;
  ELSE
    -- Status moved to absent/excused, or the rate was cleared: undo the charge.
    DELETE FROM public.ledger_entries WHERE attendance_id = NEW.id;
  END IF;

  RETURN NEW;
END $$;

DROP TRIGGER IF EXISTS trg_attendance_tuition ON public.attendance;
CREATE TRIGGER trg_attendance_tuition
AFTER INSERT OR UPDATE ON public.attendance
FOR EACH ROW EXECUTE FUNCTION public.sync_attendance_tuition();

-- Deliberately NOT backfilled. Existing attendance records stay unbilled —
-- retroactively generating charges against families' balances should be an
-- explicit decision, not a side effect of deploying this migration. Re-saving an
-- attendance record re-fires the trigger and bills that day at the current rate.