@DbRef没有在spring数据mongodb中自动保存子对象

@Document(collection = "parent")
public class Parent {
    @Id
    private String id;
    private String name;
    private Child child;

    // 构造方法、getter 和 setter 方法
}

@Document(collection = "child")
public class Child {
    @Id
    private String id;
    private String name;

    // 构造方法、getter 和 setter 方法
}

@Repository
public interface ParentRepository extends MongoRepository<Parent, String> {
    List<Parent> findByChildId(String childId);

    @Reference(to = "child")
    Child saveChild(Child child);
}