Yesterday evening I attended a session on domain-driven design (ddd) at Sogyo‘s offices, a nicely converted farmhouse located somewhere in the center of Holland. I was amazed to hear Ruby and Rails being used as an example of domain-driven development. One of the presenters even claimed that Dutch Rubyists Remco and Michiel (who reached 2nd place at this year’s RAD Race, a programming contest) had done so well because they were able to work domain-driven, thanks to Rails.

Surely he was joking. If there is one data-centric framework out there at the moment, it has to be (my beloved) Ruby on Rails. Let’s see…

• Rails’ “domain” classes (the model) are literally an implementation of the Active Record pattern, which has a direct coupling to corresponding database tables.
• As a result, Rails’ model classes correspond directly to database tables; a change in your table (e.g. an extra field) is automatically a change in your model class.
• Worst of all, DHH is actually on a mission to model the whole world under the constraints of CRUD.

I really don’t see how you can call this domain-driven. This is not to say that you couldn’t implement a domain model (as in ddd) in Ruby. But in my opinion, that is not what Rails is about. Rails is limited to the presentation layer, so its model is nothing more than a view model. For small scale applications that’s good enough. But if you need your business logic to be available independantly, to be accessible from multiple views (presentation layer applications), you’ll want to put that business logic in a separate layer, not in the model of one of those views. That is how I see “enterprise Rails”: as a tool to rapidly develop presentation side applications that use and combine existing business services (WS, EJB) (using JRuby on a Java application server).

The question remains if a domain model could be implemented in Ruby (not Rails). Technically, you could, of course. But would I want to? I’m not sure. It’s very reassuring to have the Java compiler checking your back and Eclipse’s refactoring tools at your disposal. You’d have to have some very tight unit testing to replace those and still feel safe about any changes you make. I mean, we’re potentially talking about a large company’s vital business rules here. Would I feel comfortable duck-typing those? On the other hand, there’s Ruby’s excellent DSL capabilities that would come in very handy for specifying exactly those business rules. What do you think?