As you may know Ruby 1.8.7 don't compatable with 
Rails(http://www.rubyonrails.org/down)
We recommend Ruby 1.8.6 for use with Rails. Ruby 1.8.7, 1.8.5, 1.8.4 and 
1.8.2 are still usable too, but version 1.8.3 is not.
Ruby 1.8.7 still has some bugs to straighten out.

Step-by-step to downgrade Ruby to 1.8.6 manual
http://railsgeek.com/2008/11/27/ubuntu-8-10-downgrade-ruby-1-8-7-to-1-8-6

Daniel Berger wrote:
> On Nov 26, 5:55?am, Trans <transf... / gmail.com> wrote:
>> I just updated my Ubuntu system and was a bit surprised to find:
>>
>> ? $ ruby -v
>> ? ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
>>
>> I've heard nothing but bad commentary on this version of Ruby. What's
>> the word? Should I down-grade?
> 
> I abandoned it quickly when at least one of my libraries (use) because
> of a bug. That, and Rails (for work).
> 
> Regards,
> 
> Dan

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