Breaking up Entities into smaller Entities in DDD?

Question

Does it make sense to make subsets of an Entity if you consider their usage in the application differently? IE. I take my entity and define a new entity with only some of the attributes of the first. Now I have 2 Entities that overlap but are used differently but ultimately persist in same datatable. These Entities will be accessed through different repositories...

Answer 1

I am only starting to learn about DDD myself, so if I am wrong please comment and let me know. Here are my thoughts though:

If the entity is going to be accessed through a different repository, I think it deserves its own class. Additionally, the bits that overlap now may not overlap in the future, and if you use a shared base class, you will probably be more likely to try adapting things at that point, which will dirty up your domain.

If the two classes are part of separate sub-domains, they probably should be separate. My thoughts are based around parts of an example I remember hearing in Rob Connery's interview on Hanselminutes. A product has several properties that are important to consumers (pricing, description, etc), and several properties that are important to warehouse personnel (location in the warehouse, weight, dimensions, etc). The implication to me in that episode was that the two products should be defined separately in the domain, instead of being defined once and shared.

Answer 2

If by "usage in the application" you mean you'll display different parts in different views, then I'd suggest you use a presentation pattern like Fowler describes in his Presentation Model (or if your developing a WPF app you can use the more WPF-specialized version called Model-View-ViewModel (MVVM)).

But if you by "usage" mean that you'll use different attributes of the entity in different sub-domains or part of your domain, then I agree with Chris; You'd probably be better off breaking them into different entities. The reason being that in your domain model you should reflect how the entity is used in that specific (sub)domain. And if you're using different parts of the entity under different circumstances, they probably have different meanings in these settings which again should be reflected in the naming of the entities. And if it were me, I'd probably make one repository for each of the entities. Having a 1:1 mapping between entities and repositories seems to make sense in most cases as far as I've experienced. But then again; just like Chris, Rob Conery and 90% of the developers trying to do DDD; I'm fairly new to the DDD-game and so my experiences might be overruled by someone more experienced :)