Here’s another quick tip. If you can see a gem in the list of your installed gems
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… but when you try loading it, it doesn’t work
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… make sure that you don’t have stale Gem specifications hanging around in the Gem paths. Sometimes you have a Gem installed in the system folder (/Library/Ruby/Gems/1.8), but a stale Gem specification (for example, mysql-2.8.1.gemspec) sitting in your local user Gem folder (~/.gem/ruby/1.8). Remove the stale spec and you are golden.