Ruby is 1.8.7 but still get the unsupported version error when updating rails in Leopard

Espresso:Preferences PowerBook$ ruby -v
ruby 1.8.7 (2010-08-1开发者_开发问答6 patchlevel 302) [powerpc-darwin9]
Espresso:Preferences PowerBook$ rails -v
Rails 1.2.6
Espresso:Preferences PowerBook$ gem -v
1.3.7

but when updating rails, It still get the error

Espresso:~ PowerBook$ gem update rails
Updating installed gems
Updating rails
ERROR:  Error installing rails:
    activesupport requires Ruby version >= 1.8.7.
Nothing to update

Please help

Yo,

you may use Ruby Version Manager (rvm) to manage your Rubies...

http://rvm.io/