CREATE OR REPLACE FUNCTION convert_weight(
    p_value numeric,
    p_from_unit text,
    p_to_unit text
)
RETURNS numeric
LANGUAGE plpgsql
AS $$
DECLARE
    v_from_factor numeric;
    v_to_factor   numeric;
BEGIN
    SELECT factor INTO v_from_factor
    FROM product_uom
    WHERE lower(name) = lower(p_from_unit);

    IF v_from_factor IS NULL THEN
        RAISE EXCEPTION 'Unknown source unit: %', p_from_unit;
    END IF;

    SELECT factor INTO v_to_factor
    FROM product_uom
    WHERE lower(name) = lower(p_to_unit);

    IF v_to_factor IS NULL THEN
        RAISE EXCEPTION 'Unknown target unit: %', p_to_unit;
    END IF;

    RETURN p_value * v_from_factor / v_to_factor;
END;
$$;