Domain-Driven Design

Aggregate Rules

• One repository per aggregate root
• External code only accesses aggregate through root — never child entities directly
• Aggregates reference other aggregates by ID only, not direct object reference
• Keep aggregates small — if it has more than 3-4 child entities, split it

// ✅ Aggregate root controls all access to children
order.addItem(productId, quantity); // through root
order.removeItem(itemId);           // through root

// ❌ Direct child access from outside
order.getItems().add(new OrderItem(...)); // bypasses invariants

Value Objects