November 14, 19:54
Jamis Buck has an incredible detailed exploration of how an dependency injected world would look for Rails. We could be cute and call it Rails on the Needle, but we won't — the metaphors are already fairly fuzzy.
I'm much intrigued by the whole dependency injection game. I've been following along from the sideline since JAOO '03 where I hung for a while with ThoughtWorkers Hammett and Aslak for chats about Ruby, testing, and of course Inversion of Control/Dependency Injection talks. Hammett introduced it as the Hollywood Principle and while fascinated, I wasn't really seeing the applicability at the time.
Martin Fowler's defining article on IoC Containers and the DI pattern brought all the concepts into focus, but I still wasn't really feeling the hurt that this approach would be solving in any of my Ruby work.
I am now. Writing functional tests for Basecamp is at least a little painful when there's no easy way to automatically inject mocks for my GatewayService (talking to Authorize.net) and my ActionMailer. The latter is getting souped up for better testing in the next release (so you don't even need a mock), but I'm still getting the picture. DI is great for injecting mocks in hard-to-get-to places.
But I've never been comfortable with the explicit registry lookups or method assigns. Especially for class methods, like Account.find_all. So that turned me off Jamis' first proposal. But a few talks with Dave Thomas brought up the idea of transparent DI:
So I suggested to David that we could make Rails more explicit. Rather than having some random instance variable containing the model’s class, we could declare that such-and-such a class represents a model that the controller is using, but have that class actually injected by the framework.
I like this idea a lot. It's basically business as usual with DI for free. So you wouldn't even have to choose whether to do DI or not up front. Just as soon as you're feeling the pain, you start injecting.
So I got the hots for this proposal. The major thing still holding me back is what it'll do to the complexity of the internals of Rails. I think it'll even decrease the amount of code used, but the conceptual complexity will go up. I'm just starting to get to terms with DI, so I can definitely relate to those fears that other contributors have voiced.
But I think the proposal Jamis has put together goes a long way to dispel those fears. It's centered purely around wiring controllers and models together and if you're uncertain of the value of injecting mocks, you can just not use it. And the world will spin around like it did yesterday.
Yes, I'm feeling good about it. And Jamis is about as smart a developer as they come, so when he's excited about something like Rails on the Needle, I'd be a fool not to listen really closely.
Challenge by hammett on November 14, 22:04
Hiya!
AFAIK I'm not a ThoughtWorker :-)
Its amazing, and I have to tell you that right this moment I'm working on something I called Castle on Rails (a small piece of the castle project - first draft available at http://www.castleproject.org ) which is exactly what you have described. The Castle Microkernel will inject all necessary models/services in the controller and the view...
Wow! For those who have seen the Waking Life film: once the knowledge is out there, it seems that it can be captured... :-)
Cheers!
Challenge by hammett on November 14, 22:06
Oh, and you've confused me with Paul Hammant. We both came from Avalon, maybe thats the reason :-)
Challenge by Mark on November 15, 0:14
If you're still looking for an alternative name to "Rails on the Needle", I believe you need look no further than this line of yours:
Just as soon as you're feeling the pain, you start injecting
Hence, Morphine. :)
Challenge by Rob on November 15, 0:54
Rails is getting more and more tempting all the time. I am a Java guy, and thought that DI was one of the things that Rails was missing, to see it added and so cleanly looks great.
My excuses for not doing Rails work are fast decreasing ;).
Challenge by Jamis Buck on November 15, 2:17
Morphine...heh. I can see it now. A headline: "Morphine Makes Developers Super-Productive". :-P
Challenge by William Taysom on November 15, 16:34
"transparent DI"? Sounds like Aspect Oriented Programming to me.
Challenge by John Wilger on November 16, 15:15
I'll cast my vote for adding DI to Rails. The deciding factor for me is the ability to add AOPish functionality to the model. I'm often running into instances in a current project of mine where I need to have advice added to a certain attribute assignment on a model class that doesn't really belong in the model class itself.
For instance, the project I'm working on is a PM tool for eXtreme Programming (yes, I know about Jim's StoryCards project---this is more a personal project to teach myself Ruby/Rails). In the model, I have a Project class and a StoryCard class. Project has a #status attribute that uses a third class called ProjectStatus to contain various bits of meta-data about a status. One of these is ProjectStatus#is_closed, which allows me to define several statuses but have each of them considered a closed project (i.e., Complete, Cancelled, etc.).
I want to enforce at the model level that a StoryCard can not be assigned to a Project unless Project#status.is_closed == false. However, the checking for this really shouldn't be in the StoryCard model, because that requires StoryCard to have too much internal knowledge of Project/ProjectStatus.
Currently, I'm resorting to using StoryCard.module_eval inside the Project class definition to add the status checking logic to StoryCard so that the logic is, at least, in the same file as Project. This still has a bad code smell to it, though. With DI, I should be able to use the registry to intercept the calls to StoryCard#project_id= a lot more cleanly and simply add the advice that way.
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