-- Weight-based products (e.g. fish sold per ons/kg).
-- One weighing = one order_items row: quantity stays 1, the weight goes in `weight`.

ALTER TABLE products
    ADD COLUMN sell_by VARCHAR(20) NOT NULL DEFAULT 'unit';

ALTER TABLE products
    ADD CONSTRAINT chk_products_sell_by
        CHECK (sell_by IN ('unit', 'weight'));

ALTER TABLE order_items
    ADD COLUMN weight DECIMAL(12,3),
    ADD COLUMN unit_id UUID REFERENCES units(id) ON DELETE RESTRICT;

-- A weighed line always carries a positive weight...
ALTER TABLE order_items
    ADD CONSTRAINT chk_order_items_weight_positive
        CHECK (weight IS NULL OR weight > 0);

-- ...and always represents exactly one weighing, so its quantity is pinned to 1.
-- This is what makes billable quantity unambiguous and keeps void all-or-nothing.
ALTER TABLE order_items
    ADD CONSTRAINT chk_order_items_weight_single_line
        CHECK (weight IS NULL OR quantity = 1);

CREATE INDEX idx_order_items_unit_id ON order_items(unit_id);

COMMENT ON COLUMN products.sell_by IS 'How the product is sold: unit (discrete count) or weight (weighed per transaction)';
COMMENT ON COLUMN order_items.weight IS 'Weighed amount in unit_id units; NULL for unit-priced products. Price is weight * unit_price.';
COMMENT ON COLUMN order_items.unit_id IS 'Snapshot of the product unit at sale time, so historical lines keep their meaning';