Rails development with v3 merb merger in mind

Question

I'm in the preliminary stages of designing a new web application, and have yet to begin any sort of implementation. The application models a fairly complex domain, and I'd feel more comfortable using tools such as the ruby DataMapper ORM (having using NHibernate in the .net world) than Rails Active Record. I also prefer jquery over prototype. All of these considerations of course point to using Merb, yet I'm aware that Merb is being merged into Rails for version 3 and will no longer exist as a distinct framework.

Is there any sense in starting work on the implementation of the application now given the fairly profound changes coming to rails? I'd really like to know if it would be worth starting development in Merb now and then porting it to Rails, but I've yet to find anything suggesting how difficult this may be. Another approach would be to start work on the domain now in Rails, and only give consideration to the ORM and frontend once v3 is released.

In essence, I'd like to know how portable a Merb app is going to be to Rails 3, but am aware that it may be too early for anyone other than the core developers to know this.

Any thoughts would be greatly appreciated. Thanks :)

-------------- Edit ---------------

Yehuda Katz, lead developer of the Merb project has this to say on his blog:

The plan is to start working on Rails immediately, and to continue fixing bugs and resolving other major issues in Merb in the interim. We will also release versions of Merb specifically designed to help ease the transition to Rails 3.

In particular, we will do Merb releases with deprecation notices and other transitional mechanisms to assist developers in tracking down the changes that will come between Merb 1.x and Rails 3. Expect a number of interim releases that get incrementally closer to Rails 3, and expect parts of Merb (most notably the helpers) to be ported to run on Rails 3 in order to further reduce friction.

To be perfectly clear: we are not abandoning the Merb project. There are many production applications running on Merb that are relying on both timely bug fixes and a clear path to the future. If you’re using Merb today, continue using Merb. If you’re considering using Merb for a project because it works better for your needs, use Merb. You will not be left in the cold and we’re going to do everything to make sure that your applications don’t get stuck in the past.

If you’ve already learned Merb, we will be working hard to make sure that you can parlay that knowledge into Rails 3. At Engine Yard, we fully intend to continue using Merb for our internal apps until Rails 3 is out, but we will be using those (non-trivial) applications to be sure the experience is smooth for everyone. There will be no huge jumps and you will not need to rewrite your application from scratch.

Answer 1

It's never a good time to start a complex application on a framework, really. It seems like there's always a major upgrade coming up or some other competing framework that may be a better bet. If you're having more success with Merb, stick with that right now and develop your project without fear! Both the Rails and Merb communities are going to have to cope with the merging of the projects with the release of Rails 3, but that's going to be awhile.

The project merge doesn't mean that Merb will be going away, however. Yehuda Katz will stop being the lead developer, but someone will take over the project. At the minimum, you can expect security patches and bug fixes for a few years, as long as you follow the official Merb developers' repository. Likely, after the Rails 3 release, you'll find great walk-throughs on upgrading your Rails 2.x/Merb 1.x application to Rails 3.