Fixing MySQL Gem on Snow Leopard

NOTE: See my latest post for a solution for OS X 10.6.4 and later

A fresh Snow Leopard (OS X 10.6) install does not have the mysql gem installed, and installing it fails because of missing header files:

Building native extensions. This could take a while...
ERROR: Error installing mysql:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb install mysql
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

To correctly install the mysql gem:

1. Download and install the MySQL package.
2. Install XCode that’s bundled with Snow Leopard
3. Install the gem for your architecture (use -arch i386 if you have an Intel Core Solo or Intel Core Duo processor):

# sudo env ARCHFLAGS="-arch x86_64" gem install mysql -- --with-mysql-config=/usr/local/mysql-5.1.38-osx10.5-x86_64/bin/mysql_config