Upgrading from Rails 2.3 to Rails 4.0

What you want to achieve is gonna be an epic effort. I can't provide you the step by step instructions because it's impossible to cover all the cases in a single answer.

I recommend to not upgrade Ruby and Rails in parallels, but in small steps.

The complexity of the upgrade itself is huge, but it's not impossible as long as you have a reasonable test coverage of you application. I really hope you have tests. If you don't, then you can't even think to start an upgrade like this and you first need to ensure a minimum coverage for the product.

My suggestion is to follow this roadmap

1. Upgrade Ruby from 1.8.7 to 1.9.3. Rails 2.3 is supposed to be compatible with Ruby 1.9 and you'll get the benefit of a more recent Ruby version. In fact, several gems have dropped support for Ruby 1.8.
2. Leave the app settle and run for a while, just to make sure you can find and fix incompatibilities. Every time you find an issue, reproduce it with a test, then fix it.
3. Upgrade Rails from 2.3 to 3.0.
4. Same as point 2.
5. Upgrade Rails from 3.0 to the latest 3.x version.
6. Same as point 2.
7. Upgrade Ruby from 1.9.3 to 2.0. The upgrade should be trivial.
8. Same as point 2.
9. Upgrade Rails from 3.x to the latest version

A few notes

• The reason I suggested you to not upgrade from Rails 2.3 directly to 4, it's because it's almost impossible.
• The reason I suggested you to not upgrade form Rails 2.3 directly to the latest 3.x, it's because from 3.0 to 3.x there is the asset pipeline new feature that is gonna cause you several headaches. It's better to handle this step separately and not in a big upgrade.

Ruby 1.9 handles encoding differently than Ruby 1.8. Rails 2.3 is not well prepared for this. The issues often lead to hard errors, and may occur at unexpected places (like when you want to show a error message)

A good (or scary) overview is on this blog entry: http://www.rvdh.de/2010/01/06/why-you-cant-run-rails-23-apps-on-ruby-19/

I also lost a lot of time trying to bring Rails 2.3.x to work with Ruby 1.9.x (Maybe it is possible if you are always use plain ascii)

So you need first update to Rails 3.0 and then to Ruby 1.9.3 (or both at the same time)

The asset pipeline is nice when it works, but upgrading is difficult enough. So I would just disable it until you are on Rails 4.0.

There is a lot changed with the views, like form_for helper. It is good when you have tests that render each view at least once. You then will get deprecation warning for things that have changed. If this is not realistic, then you should try to test all things of your app manually, and grep the log files for the deprecation warnings. Many deprecations in Rails 3.0 will lead to errors or wrong behaviour in Rails 3.1.

For updating the ruby and rails version to new is not take much more time but in your code you have to change all the quires and model scopes and whatever which does't work in new rails version of deprecated in new ruby and rails versions.And one more thing may be many gems which have support to ruby 1.8.7 will not support the new ruby version and same thing for the rail. My suggestions is just, you first check the dependencies of your application with new versions and the get a decisions.I have knowledge about it because i have also upgrade a full application from ruby 1.8.7, rails 2.3.5 to ruby 1.9.3 and rails 3.2.13.