require_gem改成gem的错误

require "rubygems"
require_gem "activerecord"
ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "localhost",
:database => "book",
:username => "root",
:password => "ruby",
:port => "3306"
)

class Category<ActiveRecord::Base
end

category=Category.create(
  :id=>"4",
  :name=>"ruby"
)
这样一段代码运行会提示a.rb:2:Warning: require_gem is obsolete.  Use gem instead.但上面的代码是可以写入数据库的。而当我把require_gem改成gem了就不行了，出现a.rb:3: uninitialized constant ActiveRecord (NameError)这样的错误，当然也就不能写入数据库了哈。
我的环境是rails1.2.3、ruby185-24。

require 'active_record'

ActiveRecord::......

...

谢谢了，ruby的提示真有意思，明明Use gem instead，结果却是require 'active_record'