http://blog.segment7.net/2011/02/19/rubygems-gem-activation-is-changing
We’ve changed the way gem activation works in RubyGems for the upcoming
1.6 release (at the end of the month) for RubyGems 1.5 and earlier:
$ ruby -d -S thin start
[]
Using rails adapter
[]
ExceptionGem::LoadError' at /Library/Ruby/Site/1.8/rubygems.rb:274 - can't activate rack (~> 1.0.0, runtime) for ["actionpack-2.3.5", "rails-2.3.5"], already activated rack-1.2.1 for ["thin-1.2.5"] Missing the Rails 2.3.5 gem. Pleasegem install -v=2.3.5 rails`, update
your RAILS_GEM_VERSION setting in config/environment.rb for the Rails
version you do have installed, or comment out RAILS_GEM_VERSION to use
the latest version installed.
With RubyGems from trunk:
$ ruby -I ~/Work/git/rubygems/lib -S thin start
Using rails adapter
Thin web server (v1.2.5 codename This Is Not A Web Server)
Maximum connections set to 1024
Listening on 0.0.0.0:3000, CTRL+C to stop
RubyGems now lazily activates gems so a gem’s load paths will not be
added to $LOAD_PATH until its files are needed.
We would like testers for our changes to gem activation. If you would
like to help us test you will not be able to release gems. Gem authors
should be prepared to downgrade to RubyGems 1.5.2 if you test the trunk
version.
If you wish to test the new version of RubyGems you will need to check
out the repository using git fromgit://github.com/rubygems/rubygems.git
then run ruby setup.rb. You can also test without upgrading by usingruby
-I path/to/rubygems/lib. If you’re running a ruby executable such as
thin run ruby -I path/to/rubygems/lib -S thin.
Please report any issues you have on the mailing list (subscribe here)
or in the #rubygems channel on chat.freenode.net(check that drbrain is
active).
When you’re done testing you can downgrade to RubyGems 1.5.2 using the
Downgrade Recipe in the UPGRADINGdocument.