领域驱动设计，实体调用存储库来验证相关资源

Question

我正在尝试在 php 中实施 DDD。我有一个名为 message 的实体，它与 fromFieldId 相关。我想验证这个 fromField 在消息上设置时是否存在于数据库中，我不想给调用者增加这种验证的负担，因为它在我的代码中的很多地方都会发生。但是据我所知，DDD 实体不应该调用存储库吗？我将如何在 DDD 中处理这个问题？

我想要这样的东西：

class MessageEntity{

     public function setFromFieldId($fromFieldId){
         if(!$this->fromFieldRepository->isExists($fromFieldId)){
              // throw some exception
         }
         $this->fromFieldId = $fromFieldId;
     }

 }

Answer 1

您可以简单地从存储库加载 FromField AR 并将整个实例传递给 Message AR，但只保留 ID。

伪代码：

MessageApplicationService {
    setFromFieldId(messageId, fromFieldId) {
        fromField = fromFieldRepository.findById(fromFieldId);

        //You could have a null check for fromField here
        //or call a method that throws automatically
        //e.g. findExistingById(). You could also prevent
        //null from being passed in message.setFromField

        message = messageRepository.findById(messageId);
        message.setFromField(fromField); //only holds onto the id
    }
}

Answer 2

我想你需要的是DomainService DDD。

来自 Eric Evans 领域驱动设计：

When a significant process or transformation in the domain is not a natural responsibility of an ENTITY or VALUE OBJECT, add an operation to the model as standalone interface declared as a SERVICE. Define the interface in terms of the language of the model and make sure the operation name is part of the UBIQUITOUS LANGUAGE. Make the SERVICE stateless.

在您的情况下，如果仅设置一次字段在您的通用语言中是一个正确的概念，那么一种方法可以是：

class SettingFieldOnceService
{
    private $repository;

    public function __construct(Repository $repository)
    {
        $this->repository = $repository;
    }

    public function setFieldInEntity($field, $entity)
    {
        if ($anotherEntity = $this->repository->findByField($field)) {
            throw new DomainException("...");
    }

        $entity->setField($field);
    }
}

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