DDD with ORM where should the business logic go?

Question

I have used an MDA (model driven architecture) tool in the past where we modeled via UML and this generated the business entities (our domain model) and the ORM (mapping etc) amongst other things.

A lot of the business code and services working on the domain were part of the model and our repositories were returning the business entities (so it would have been impossible to switch out to another ORM (not that we wanted to)).

However, now I am starting a project and I want to think in terms of DDD.

So far it feels as though I put my business logic in my domain model and via repositories I would work with the ORM (which ever I chose). However, if I wanted to continue to use the MDA tool for the ORM part of the application, the model created here would be very anemic (i.e not contain any business logic). Similarly if I used Entity framework (.net) or NHibernate for my ORM it too would be an anemic model.? I am not sure where you would put the business logic if I just used NHibernate.

Am I correct in thinking this way, in other words with DDD all the business logic in the domain and just use the ORM for persistence via repositories?

Answer 1

so it would have been impossible to switch out to another ORM (not that we wanted to)).

That seems wrong. A major advantage of the repository pattern is that you hide the data access logic and that it is easily exchangeable.

So far it feels as though I put my business logic in my domain model and via repositories I would work with the ORM (which ever I chose). However, if I wanted to continue to use the MDA tool for the ORM part of the application, the model created here would be very anemic (i.e not contain any business logic). Similarly if I used Entity framework (.net) or NHibernate for my ORM it too would be an anemic model.? I am not sure where you would put the business logic if I just used NHibernate.

An anemic domain model is considered a bad practice by many, for example by Martin Fowler. You should avoid such a design because such a model leads to procedural design techniques rather than a good object oriented design. You then have data classes and manager/processing classes which means you separated state and behaviour. But an object really should be "state and behaviour".

NHibernate does a great job at persistence ignorance. You can hide away the mapping details in XML or with FluentNHibernate and just write plain POCOs. It's very easy to create a rich domain model with NHibernate. I think you can do that with entity framework and the MDA tool, too. As long as this tool produces partial classes you can extend the generated code pretty easily without having to worry that a new generation might destroy your user-written code.

To cut this long story short. When you use NHibernate, nothing, I repeat nothing, stops you from embracing a rich domain model. I recommend using it with FluentNHibernate and mapping by hand. The mapping code takes only 5 to 10 minutes to write. I suppose that the very same is true for entity framework and that its tools at least create partial classes that are easily extensible.

Am I correct in thinking this way, in other words with DDD all the business logic in the domain and just use the ORM for persistence via repositories?

For the most part you are correct. You should have a rich domain model. Especially when things become more and more complex, it's easier to maintain and extend when you've designed it properly. But do keep in mind that DDD also knows (Domain Layer and Application Layer) Services to implement business logic and Factories to encapsulate creational logic.

I also tend to differentiate business logic into domain logic and actual application business logic. The domain logic is how the domain interacts and behaves while the application logic, which is a completely different layer, encapsulates how the domain is used for the specific use-case/application. Often times I have to update the domain model to support specific use cases and to make it more powerful.

Answer 2

However, if I wanted to continue to use the MDA tool for the ORM part of the application, the model created here would be very anemic (i.e not contain any business logic).

I don't know which MDA tool you're using, but the ones I've worked with always create partial classes so you're free to complete them with as much business logic as you want.

However, I do feel MDA tools are a little bit less appropriate in a DDD context than ORMs, because code generation often tends to produce classes that are muddled with tool-specific noise instead of the streamlined, clearly expressed domain entities we expect. Actually, what you often get is a mix of domain data, persistence logic, constraint validation logic... and I don't know if there's a way to separate these concerns with most MDA tools.

And of course, you can't touch the generated code except via partial classes, which means you can't eliminate potential anti-DDD behaviour that would be integrated. This is problematic in an approach where you want to enforce strict barriers between Aggregates and tailor the relationships between your entities perfectly. Build time in a continuous integration environment can suffer from the additional code generation step as well.

Apart from that, I think Falcon pretty much said it all -absolutely nothing in ORM or MDA tools prevents you from having rich domain entities.

Answer 3

What I do in my team is to model my object, domain and add my business logic at the same time. I don't use Model Driven Development which would generate a code from a model but prefer the annotation approach. I mean that at object level inside the class diagram I add ORM stereotypes. This will add persistence annotations directly in the code which are compatible with EJB3/hibernate. THe database creation is done by Hibernate and certainly not by the UML templates. This is a lot better because if the code change and the added annotations is not exactly what the hibernate specialist then he/she can change it but the model is still good. I can also change my requirements and my domain model is still ok.

Developers can add business logic inside each method and add a comment, I can also model and add constraints. For example the sales should be over 50k if not etc.... I don't need to code it but just write in the model and this information woould be visible to the developer team. Really cool and flexible UML.