Can't require datamapper

I have ruby up to date but can't require datamapper:

    $ gem -v
    1.8.11
    $ gem list |grep datamapper
    datamapper (1.2.0)
    $ ruby -v
    ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]
    $ irb
    irb(main):001:0> require "datamapper"
    LoadError: cannot load such file -- datamapper
        from
/usr/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from
/usr/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from (irb):1
        from /usr/bin/irb:12:in `<main>'
    irb(main):002:0>

do you know what's happening?
i'm using archlinux 64bit

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Ok forgot it, they changed it to "data_mapper".

Nevermind

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