error: undefined method `gem` for main:Object

[stardiviner]

ri_vim.rb:5: undefined methodgem' for main:Object (NoMethodError)`

I'm new on Ruby, I do not know which part I miss, and I have not installed something.
In README, ri.vim need rdoc 3.8, but I have not found this version.
In my Ubuntu 12.10 package system, it has rdoc1.8 and rdoc1.9, and I searched gems with $ gem search rdoc, have not found rdoc3.8 too, only has version rdoc (3.12).

[stardiviner]

I have executed command # gem install rdoc --version 3.8 but still get this error.

[danchoi]

Are you launching Vim with Ruby 1.8 or Ruby 1.9 active? You can see which one is active by typing :! ruby -v in Vim.

[stardiviner]

I solved this problem, I searched on google how to let vim use ruby 1.9, then I found removing old ruby 1.8 can solve this problem.