DDD - the rule that Entities can't access Repositories directly

Question

In Domain Driven Design, there seems to be lots of agreement that Entities should not access Repositories directly.

Did this come from Eric Evans Domain Driven Desi

Answer 1

Its a very good question. I will look forward to some discussion about this. But I think it's mentioned in several DDD books and Jimmy nilssons and Eric Evans. I guess it's also visible through examples how to use the reposistory pattern.

BUT lets discuss. I think a very valid thought is why should an entity know about how to persist another entity? Important with DDD is that each entity has a responsibility to manage its own "knowledge-sphere" and shouldn't know anything about how to read or write other entities. Sure you can probably just add a repository interface to Entity A for reading Entities B. But the risk is that you expose knowledge for how to persist B. Will entity A also do validation on B before persisting B into db?

As you can see entity A can get more involved into entity B's lifecycle and that can add more complexity to the model.

I guess (without any example) that unit-testing will be more complex.

But I'm sure there will always be scenarios where you're tempted to use repositories via entities. You have to look at each scenario to make a valid judgement. Pros and Cons. But the repository-entity solution in my opinion starts with a lot of Cons. It must be a very special scenario with Pros that balance up the Cons....