Using Rails 2.1.1, I get this warning about the mysql libraries associated with Rails:

The bundled mysql.rb driver has been removed from Rails 2.2. Please install the mysql gem and try again: gem install mysql.

As many have noted, the suggestion of `gem install mysql` doesn't seem to work well on the vanilla Leopard install with the standard MySQL pre-compiled binary. I followed lots of good advice from wonko, jlaine, hivelogic, and Seth Willits, but finally none of it worked.

Using the command from wonko:

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

got me the ulong compile error mentioned by jlaine,. Unfortunately, jlaine's instructions didn't work for me, the manual install step doesn't seem to really install anything. I realized I had to take it step further by modifying the source of he gem itself.

In order to fix the compile error in mysql.c, it helps to understand how the gem install command works. First, gem goes online to find the correct file, then downloads the packaged .gem file to your local gem cache. Then, gem unpacks the .gem and runs extconf.rb and make/make install. From reading extconf.rb, one can see that the mysql gem uses a file called mysql.c.in as a template to build a properly configured mysql.c, which then gets compiled into the C library that we will use.

$cd /Library/Ruby/Gems/1.8/cache
$sudo gem unpack mysql-2.7.gem
$cd mysql-2.7

This gives you the source of the gem. Open mysql.c.in to add the single line we need.

#define ulong unsigned long // quick fix so that it compiles on my machine

Then we need to repack the gem, copy it to the cache folder, and install our customized version using the command above.

$sudo gem build mysql.gemspec
$sudo mv mysql-2.7.gem ../mysql-2.7-CUSTOM.gem
$sudo env ARCHFLAGS="-arch i386" gem install mysql-2.7-CUSTOM.gem -- \
  --with-mysql-dir=/usr/local/mysql --with-mysql-lib=/usr/local/mysql/lib \
  --with-mysql-include=/usr/local/mysql/include

All this for one line of code...but finally:

$ gem list mysql

*** LOCAL GEMS ***

mysql (2.7)

Thankfully, most of the time we don't need to go through all this just to instal a library. This sort of thing happens rarely. But its great to know that as long as you have your command line, a good compiler, and some detective skills, you can fix many problems on your own!