record Student(
	Identity id,
	String name,
	List<Course> courses) {
}

record Course(
  Identity id,
  String name,
  List<Student> students) {
}
record Student(
	Identity id,
	String name,
	List<Course> courses) {
}
record Course(
	Identity id,
	String name) {
}

// ある学生が履修している講座の一覧を取得する
student.courses()
record Student(String name) {
}
record Course(String name) {
}

record Enrollment(Student student, Course course) {
}
record Student(
	Identity id,
	String name,
	List<Identity> courseIds) {
}

record Course(
	Identity id,
	String name,
	List<Identity> studentIds) {
}

void enroll(Student student, Identity courseId) {
}

// ある学生が受講している講座一覧を取得する
// 必要に応じて実体を取得する
List<Course> listCourse(Long studentId) {
	return select().from("courses")
		.join(table("enrollments"))
		.on(table("enrollments").field("course_id").eq(table("courses").field("id")))
		.where(table("enrollments").field("studentId").eq(studentId))
		.fetch()
		.map(courseMapper::toDomain)
}
record Course(
	Identity id,
	String name,
	List<Student> students) {
}
record Course(
	Identity id,
	String name,
	List<Student> students) {  // null or loaded
}
record Course(
	Identity id,
	String name,
	List<Student> students) {
	
	public List<Student> students() {
		if (this.students instanceof VirtualList vl) {
			vl.load();
		}
		return this.students;
	}
)
record Restaurant(
	Identity id,
	String name,
	List<Table> tables
) {}

record Table(
	Identity id,
	int tableNumber,
	int capacity
){}

void save(Restaurant restaurant) {
	updateRestaurant(restaurant);
	// 
	save
}
record User(Identity id, String name) {}

record Group(Identity id, String name) {}

record Membership(User user, Group group) {}
record Group(List<User> users) {
	void join(User user) {
		if (users.size() > 10) {
			throw new OverException(10);
		}
		users.add(user);
	}
}
Membership join(Group group, User user) {
	int numOfMembers = groupRepository.countMembers(group.id());
	if (numOfMembers > 10) {
		throw new OverException(10);
	}
	return Membership.of(user, group);	
}
record Order(Identity id,
	Customer customer,
	Product product,
	LocalDateTime orderedAt) {}
if (!productRepository.exists(req.productId())) {
	throw new ProductNotFoundException(req.productId);
}
// ...
Optional<Product> product = productRepository.findById(req.productId());

product.ifPresent(p -> {
	Order order = new Order(
		id, customer, product, orderedAt
	);
	orderRepository.save(order);
});
record Order(Identity id,
	Customer customer,
	List<OrderLine> lines
	LocalDateTime orderedAt) {}

record OrderLine(
	Product product,
	int amount
) {}
record Order {
	Identity id,
	LocalDateTime orderedAt
}

record Invoice {
	Identity id,
	Identity orderId,  // 依存する側が参照を持つ
	LocalDateTime invoicedAt
}
record Order {
	Identity id,
	LocalDateTime orderedAt
}

record Invoice {
	Identity id,
	List<Identity> orderIds,  // 請求が複数の注文に依存
	LocalDateTime invoicedAt
}