I've had issues installing the ruby mysql gem a couple of times, so I thought I'd document what finally worked here. I had Rails 2.3.8 running on the pre-installed Ruby 1.8.7 that comes with OS X 10.6. I needed to install Rails 3.x for another project. Having multiple versions of Rails is supposed to work OK, so I just did:
sudo gem install rails
Problems began here, but I flailed rather than keeping careful notes. At some point, I ended up thinking an fresh install of MySQL might help, so I installed version 5.5.9 from the DMG on dev.mysql.com. Maybe, I shoulda used MacPorts, cause that made things worse. After that, neither version of rails could connect to MySQL and my cubical-neighbors had new respect for my colorful vocabulary. Rails would fail, croaking up this cryptic message:
uninitialized constant MysqlCompat::MysqlRes
This thread helped. You need to be careful that MySQL, Ruby and MySQL/Ruby are compiled to a common architecture. You can do this by specifying ARCHFLAGS in the environment.
sudo env ARCHFLAGS="-arch x86_64" gem install --no-rdoc --no-ri mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
Compiling the gem properly is one step. The gem depends at run time on the mysql client library, which it needs to be able to find. Specify that, like so:
With the combination of these two clues, both versions of Rails seem to work happily.
Diagnosing problems with Ruby and Gems
A couple key commands for debugging RubyGems are gem list and gem env. Also, gem uninstall for removing the wreckage of failed attempts. Some recommend RVM, which I may try out some time. Also, I use the deprecated practice of installing gems with sudo. I guess I should learn to install them in my user directory.
The Ruby that comes with OS X 10.6, at least on my machine, looks like this:
$ /usr/bin/ruby --version ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0] $ file /usr/bin/ruby /usr/bin/ruby: Mach-O universal binary with 3 architectures /usr/bin/ruby (for architecture x86_64): Mach-O 64-bit executable x86_64 /usr/bin/ruby (for architecture i386): Mach-O executable i386 /usr/bin/ruby (for architecture ppc7400): Mach-O executable ppc
Installing a fresh Ruby from MacPorts probably wasn't necessary, but that didn't stop me:
$ which ruby /opt/local/bin/ruby $ /opt/local/bin/ruby --version ruby 1.8.7 (2011-02-18 patchlevel 334) [i686-darwin10] $ file /opt/local/bin/ruby /opt/local/bin/ruby: Mach-O 64-bit executable x86_64
I hope this helps someone. This is a pain, but the same thing in Python is worse.