fuleis-MacBook-Air:etc cow$ gem install mysql2 /Users/cow/.rvm/rubies/ruby-1.9.3-p362/lib/ruby/1.9.1/yaml.rb:56:in `': It seems your ruby installation is missing psych (for YAML output). To eliminate this warning, please install libyaml and reinstall your ruby. Fetching: mysql2-0.3.11.gem (100%) Building native extensions. This could take a while... Successfully installed mysql2-0.3.11 1 gem installed Installing ri documentation for mysql2-0.3.11... Installing RDoc documentation for mysql2-0.3.11...