Invoice INV-000001
| Item | Unit price | Qty | Line |
|---|---|---|---|
| Nanosecond (11.8 inches of wire) | $4.00 | 20 | $80.00 |
| On Computable Numbers (annotated) | $32.00 | 1 | $32.00 |
An invoice is its own entity — number, total, issue time and billing address are all copies — and order.invoice is a has_one because the invoice's relates_to :order is unique: true.