Here’s how to get the latest version of MySQL (5.0.51b as of this writing) working with the MySQL/Ruby gem and the Apple-supported version of Ruby that comes with Mac OS X Leopard. These instructions are scattered around the Internets in various places, but I’m sick of searching for them every time I need them so I’m compiling them here in a single place for convenience.
First, download and install MySQL for Mac OS X 10.5 (x86). Don’t install the x86_64 build or Ruby will refuse to speak to it. If you’ve already installed the x86_64 build, backup your databases, install the x86 build on top of it, and restore your databases.
Once you’ve got the correct build of MySQL installed, pop open a terminal and run the following to install the MySQL/Ruby gem:
sudo env ARCHFLAGS="-arch i386" gem install mysql -- \
--with-mysql-dir=/usr/local/mysql --with-mysql-lib=/usr/local/mysql/lib \
--with-mysql-include=/usr/local/mysql/include
That should do the trick.