-- Deferred reconciliation permits nested line creation in the same transaction
-- while preventing incomplete orders or later additions to an existing snapshot.
CREATE FUNCTION reconcile_order_lines() RETURNS trigger LANGUAGE plpgsql AS $$
DECLARE
  target_order UUID;
  expected NUMERIC;
  actual NUMERIC;
BEGIN
  IF TG_TABLE_NAME = 'orders' THEN target_order := NEW.id;
  ELSE target_order := NEW.order_id;
  END IF;
  SELECT subtotal INTO expected FROM orders WHERE id = target_order;
  SELECT COALESCE(SUM(line_total), 0) INTO actual FROM order_lines WHERE order_id = target_order;
  IF expected IS DISTINCT FROM actual THEN
    RAISE EXCEPTION 'Order subtotal does not match lines';
  END IF;
  RETURN NULL;
END;
$$;
CREATE CONSTRAINT TRIGGER orders_reconcile_lines AFTER INSERT ON orders
DEFERRABLE INITIALLY DEFERRED FOR EACH ROW EXECUTE FUNCTION reconcile_order_lines();
CREATE CONSTRAINT TRIGGER order_lines_reconcile_total AFTER INSERT ON order_lines
DEFERRABLE INITIALLY DEFERRED FOR EACH ROW EXECUTE FUNCTION reconcile_order_lines();