关于领域驱动设计与开发过程中的一些疑惑请道友帮忙解惑，谢谢。

public class Car{
   private Wheel wheel;
   private Engine engine;

   ..
}
<p class="indent">

CREATE TABLE car(
  carID       BIGINT NOT NULL,
  name          VARCHAR(100) NOT NULL,
  PRIMARY KEY   (carID)
);

CREATE TABLE wheel(
  wheelID       BIGINT NOT NULL,
  name          VARCHAR(100) NOT NULL,
  PRIMARY KEY   (wheelID)
);

CREATE TABLE engine(
  engineID       BIGINT NOT NULL,
  name          VARCHAR(100) NOT NULL,
  PRIMARY KEY   (engineD)
);

<p class="indent">