MacOS X: active_record module doesn't load

When I type

require 'active_record"

in IRB, I get this:

NoMethodError: undefined method cattr_accessor' for ActiveRecord::Base:Class from /usr/lib/ruby/site_ruby/1.8/active_record/base.rb:246 from /usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:ingem_original_require’
from
/usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in require' from /usr/lib/ruby/site_ruby/1.8/active_record.rb:37 from (irb):2:inrequire’
from (irb):2

What do I need to do to get this to load?