I wanted to use the rake gem in it version 0.8.7 but in my rvm was also installed a 0.9.2 version.

I tryed to uninstall it with no success and the gem ask to run the

gem list -d rake

 rake (0.9.2, 0.8.7)
 Author: Jim Weirich
 Rubyforge: http://rubyforge.org/projects/rake
 Homepage: http://rake.rubyforge.org
 Installed at (0.9.2): /Users/spleenteo/.rvm/gems/ruby-1.8.7-p330@global
 (0.8.7): /Users/spleenteo/.rvm/gems/ruby-1.8.7-p330

wtf is that @global? so, the only way to remove it is:

$ rvm use @global
$ gem uninstall rake
$ rvm use ree

this worked fine for me.

It might be a problem, an “old” well-known issue…

sudo gem install mysql

might return an error like:

ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)

    ERROR: Failed to build gem native extension.

Well, this is cause you need the -dev environment of mysql. Too easy to think that it could be mysql-dev, no! just try

apt-get install libmysqlclient15-dev

It will works!