域实体应该调用存储库吗？

Question

我正在设计一个运输应用程序，并尝试使用Clean Architecture和DDD。在域层的核心深处，我们有许多可配置的业务规则。例如，有一些业务规则用于确定货运的最佳承运人，确定运输方式，确定付款类型等。每个业务规则都会从数据库中选择数据，因此我打算使用BizRule存储库。问题是，根据我对DDD原理的理解，域实体（例如，货件）不应调用存储库（例如，BizRuleRepository）。用例层应该是调用存储库的那一层。如果我采用这种方法，那么我将不得不将许多复杂的业务规则移至用例层，我​​不确定这是否是最佳方法。在这种情况下，设置例外并让域实体调用存储库是否有意义？先感谢您。

Answer 1

域实体应该调用存储库吗？

Generally speaking, no; it doesn't make sense for an entity (which is a domain concern) to be communicating directly with a repository (which is plumbing).

Evans, when organizing his book, assigned these ideas to different chapters

• A Model Expressed in Software -- describes Entities
• The Life Cycle of a Domain Object -- describes Repositories

It's a problem of language; Repositories normally have collection or persistence semantics, which are not (typically) part of the ubiquitous language of the domain.

That said, there is a loop hole; domain services can describe data retrieval using the ubiquitous language, and delegate that work to application or infrastructure services.

So (assuming for the moment that the business rules are a domain concept), you would have a domain entity that knows which business rule it needs, and a domain service that knows how to retrieve a business rule, and then the entity that knows how to use it.

If business rules are not a domain concept, then some of the work shifts from the entity into the domain service, but the core of the pattern remains the same -- the entity passes arguments to the service, the service returns a domain value the entity understands, the entity decides how to apply that value in its current processing.

We can solve any problem by introducing an extra level of indirection

有点壳游戏；在幕后，我们仍在使用管道；但是领域模型只能看到瓷器。

当您要对域逻辑进行单元测试而又不拖累整个管道依赖关系时，这种额外的间接层可能真的很方便：您将域服务替换为测试倍数。