Freeze what the customer agreed to on the order

How to do it

Prices, tax rates and coupons change. An order, its invoice and its refunds must show what the customer agreed to. Keep the price ID on the line for reports. Store the tax calculation ID from your tax service too. Refunds and credit notes work only from the stored lines.

Example

CREATE TABLE order_lines (
  order_id          text NOT NULL REFERENCES orders(id),
  line_no           int  NOT NULL,
  price_id          text NOT NULL,
  description       text NOT NULL,
  quantity          int  NOT NULL CHECK (quantity > 0),
  unit_amount_minor bigint NOT NULL,
  discount_minor    bigint NOT NULL DEFAULT 0,
  tax_rate_bp       int  NOT NULL,
  tax_minor         bigint NOT NULL,
  currency          char(3) NOT NULL,
  PRIMARY KEY (order_id, line_no)
);

How to check

On staging, change a price and a tax rate, then open an older order and refund it. The order, the invoice and the refund all show the old amounts.