Thinking of taking up coffee.

I hate computers.

$ gem install spree
ERROR:  Error installing spree:
	actionpack requires rack (~> 1.0.0, runtime)

$ gem install rack
Successfully installed rack-1.1.0
1 gem installed

$ gem install spree
ERROR:  Error installing spree:
	actionpack requires rack (~> 1.0.0, runtime)

$ gem install rack -v=1.0.0
Successfully installed rack-1.0.0
1 gem installed

$ gem install spree
...
26 gems installed

$ rake gems:install
Missing the Spree 0.10.2 gem. Please `gem install -v=0.10.2 spree`, update your SPREE_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out SPREE_GEM_VERSION to use the latest version installed.

[commented out the SPREE_GEM_VERSION line]

$ rake gems:install
Missing the Spree  gem. Please `gem install -v= spree`, update your SPREE_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out SPREE_GEM_VERSION to use the latest version installed.

[uncommented "SPREE_GEM_VERSION = '0.10.2'" again]

$ gem install -v=0.10.2 spree
Successfully installed spree-0.10.2
1 gem installed

$ ./script/server 
Missing the Spree 0.10.2 gem. Please `gem install -v=0.10.2 spree`, update your SPREE_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out SPREE_GEM_VERSION to use the latest version installed.

$ spree derp
/Library/Ruby/Site/1.8/rubygems.rb:233:in `activate': can't activate rack (>= 1.0.1, runtime) for ["spree-0.10.2"], already activated rack-1.0.0 for ["actionpack-2.3.5", "rails-2.3.5", "spree-0.10.2"] (Gem::LoadError)
	from /Library/Ruby/Site/1.8/rubygems.rb:249:in `activate'
	from /Library/Ruby/Site/1.8/rubygems.rb:248:in `each'
	from /Library/Ruby/Site/1.8/rubygems.rb:248:in `activate'
	from /Library/Ruby/Site/1.8/rubygems.rb:1082:in `gem'
	from /Users/inky/.gem/ruby/1.8/bin/spree:18

$ gem uninstall rack -v=1.1.0

Select gem to uninstall:
 1. rack-1.1.0
 2. rack-1.1.0
 3. All versions
> 2

You have requested to uninstall the gem:
	rack-1.1.0
actionpack-2.3.7 depends on [rack (~> 1.1.0)]
spree-0.10.2 depends on [rack (>= 1.0.1)]
If you remove this gems, one or more dependencies will not be met.
Continue with Uninstall? [Yn]  n
ERROR:  While executing gem ... (Gem::DependencyRemovalException)
    Uninstallation aborted due to dependent gem(s)

$ shutdown

Made me laugh because that's exactly how my last 3 days have been. Thanks for sharing your pain. :)

google brought me to you. throw rvm into the mix and you have a spree party!

so any news from you ?

yup - it is a pain. on the bright side, bundler or rvm gemsets can solve this problem.
but you have to find out and define the dependencies once - it only manages the dependency. we do not have to go through the pain a second time.

also you should have tried

rake gems:install
at first

that way u would not have installed spree and then removed it, the rake task would have taken care to install the proper dependency.
also i never manually install a lower dependency in a project, install it in a rvm gemset.
know from personal experience that upgrading rack breaks stuff as it is used in a lot of places.
rails + rack
merb + rack
resque + rack
passenger + rack
+ pack

do not manually install rack and other smaller libs other than in a gemset and after making sure the dependencies are defined in ur project and the app starts up, lock the bundle and check it in git

and then have a coffee :-)

another good practice is to have a rvm gemset per-project u are working on.

http://rvm.beginrescueend.com/workflow/rvmrc/

echo "rvm ree@projectb" > ~/projects/projectb/.rvmrc

this segregates the gems quite nicely, one project will not break due to another project.