-- migrate:up

-- A reservation names either a unit we know (Whiskey group, `unit_id`) or a
-- free text the requester typed (`unit_label`). Typing a name must never create
-- a `units` row, so the two are exclusive rather than the second being a
-- fallback name for the first.
ALTER TABLE reservations ALTER COLUMN unit_id DROP NOT NULL;
ALTER TABLE reservations ADD COLUMN unit_label TEXT;

ALTER TABLE reservations ADD CONSTRAINT reservations_unit_xor CHECK (
    (unit_id IS NULL) <> (unit_label IS NULL)
);

-- A free label is a name, not an empty string
ALTER TABLE reservations ADD CONSTRAINT reservations_unit_label_not_blank CHECK (
    unit_label IS NULL OR btrim(unit_label) <> ''
);

-- migrate:down

ALTER TABLE reservations DROP CONSTRAINT reservations_unit_label_not_blank;
ALTER TABLE reservations DROP CONSTRAINT reservations_unit_xor;

-- The rows that only had a free label have no unit to point at any more
DELETE FROM reservations WHERE unit_id IS NULL;
ALTER TABLE reservations DROP COLUMN unit_label;
ALTER TABLE reservations ALTER COLUMN unit_id SET NOT NULL;