عرض عملي: ربط Order وOrderLine كتجميع (Aggregate)

خطوة بخطوة

1. Create Order (id, customer, status, createdAt) and OrderLine (id, product, quantity, unitPrice).
2. Add @ManyToOne(LAZY) from Order to Customer and from OrderLine to Product.
3. Add lines with cascade = ALL, orphanRemoval = true and the addLine/removeLine helpers.
4. Planned mistake: add a line to the list without setting line.setOrder(this); show the null foreign key error and fix it with the helper.
5. Save an order and read the SQL log: one order insert, then one insert per line.