带有@Version的Spring-data-mongodb DbRef延迟导致org.springframework.dao.DuplicateKeyException

Question

Spring-boot 2.1.4， Spring-Data-Mongo 2.1.6， 加载并重新保存延迟加载的内容时，出现DuplicateKeyException。

@Document
public class User {
   @Id
   private String id;
   @Version
   private Long version;
   @DBRef(lazy = true)
   private Role role;
}

@Document
public class Role {
    @Id
    private String id;
    @Version
    private Long version;  
}
User user = userRepository.findById("1234");
roleRepository.save(user.getRole()); //org.springframework.dao.DuplicateKeyException

它无需lazy = true即可工作