... or how to spend unneccessary time on infrastructure problems.
Ruby is a great language and offers many tools to quickly develop (web) applications and awesome libraries. But sometimes it sucks. And that is the case when migrating applications to new Ruby versions.
The last weekends I worked on switching my Ruby on Rails 2.x applications from Ruby 1.8 to Ruby 1.9. One thing I learned is that Ruby 1.9 uses UTF-8 for file encoding now. Rails 2.3 applications and libraries (e.g. the mysql gem) have a number of issues with that, different ones depending on the platform you are running ruby on. This is a non-complete list of things to care about when updating.
Encoding Update
To start with, Ruby uses the LANG environment variable to determine file encoding. So make sure you have it set to a UTF-8 locale on your system.
irb(main):001:0> ENV['LANG']
=> "en_US.UTF-8"
Next, make sure that you do not use any special NON-ASCII characters (åößè are good candidates) in mail templates because the action-mailer gem for Rails 2.3 does not quite seem to understand UTF-8 yet.
Also, make sure your source files are UTF-8 encoded (adding the # encoding: utf-8 header may be required, too). And then, instruct your rails application to use UTF-8 as the default encoding:
# add to top of config/environment.rb
Encoding.default_external = Encoding.default_internal = Encoding::UTF_8
If you use regular expressions, do not wonder why they don't match anymore. They just don't because you may not have solved all encoding problems yet.
After encoding
You are done with encoding problems now and want to use all the latest UTF-8 compatible gems now?
Reconsider. Then make sure to stick to the 1.x version of rubygems. Because rails 2.3 does not like the 2.0 versions of the gem.
Check your database connectivity library (I used the mysql gem for Ruby 1.8). The mysql gem has encoding issues on Linux systems, so switching to the mysql2 gem is inevitable. Only versions < 0.3 work though. Just because. If you are on windows stick to the mysql gem for now. Because mysql2 gives Bad file descriptor errors.
Remove deprecation warnings
Use Object#tap instead of #returning. Looks awful in code but prevents your server log from getting spammed.
Think about monkey patching your Rails 2 app for some more UTF-8 compatibility or switch directly to Rails 3.
Finally
Drink a coffee, eat a muffin and read about others going through the same kind of stuff, some even spending month to do it.