Aggregate Root calling other aggregate root domain service.

Question

I have an Aggregate Root (AR1) that references another Aggregate Root (AR2) by Identity. i.e. as discussed here http://www.informit.com/articles/article.aspx?p=2020371&seqNum=4

Now in one of my methods in AR1 I need to access the full object of AR2 to perform a check. I have created a domain service called AR2DomainService and that is now being injected into the method of AR1. For example:

public class AR1 
{
     public Guid AR2Id; 

     public void DoSomething(IAR2DomainService, aR2DomainService)
     {
        AR2 ar2 = ar2DomainService.GetById(Ar2Id);

        if(ar2.status != Status.OK)
            //throw exception

        //do some processing. 

     }

     public bool DomSomething2(IAR2DomainService, ar2DomainService)
     {
          return ar2DomainService.DoSomething(Ar2Id); 
     }


}

Is this code ok?

Answer 1

As plalx points out, Domain Services aren't here to retrieve aggregates. What I would do is coordinate everything from the Application Service/Command Handler. It reads the associated Aggregate Root ID from AR1, and retrieves it through a Repository. Then it can extract relevant information from AR2 and pass it as a parameter to AR1's method. Note that the smaller this parameter, the better. Ideally you wouldn't pass AR2 entirely but only a Value Object containing the minimum information needed (you don't want to be tempted to update AR2 as part of a transaction in AR1).