Rails 2.x no longer contains the “mysql” adapter. Fine, then I just install it from rubygems:

# sudo gem install mysql

But it fails with

*** extconf.rb failed ***

First fix: tell the gem installer where my mysql installation sleeps:

# sudo gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config

This time it installs fine, except that I get errors when I try to use it:

LoadError: dlsym(0x1412860, Init_mysql): symbol not found - /opt/local/lib/ruby/gems/1.8/gems/mysql-2.7/lib/mysql.bundle

After some research, the problem is that I installed the 64bit version of mysql and the gem expects the 32bit version. Went to the downloads page, installed x86 instead of x86_64 (warning : this will destroy all your databases !!) and retried:

# sudo gem uninstall mysql
# sudo gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
# irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> require 'mysql'
=> true

Hope that helps someone with the same kind of setup.