Can't find gems in irb: "NameError: uninitialized constant Gem from (irb)"

Question

When I run a Rails app it finds all of my installed gems correctly.

This is the first time I tried to call some gems from inside irb and it couldn't find them.

blocke:~$ irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> require 'rails'
LoadError: no such file to load -- rails
 from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 from (irb):2
irb(main):003:0> 

When I run "gem environment" this is what I get:

blocke:~$ gem environment
RubyGems Environment:
  - RUBYGEMS VERSION: 1.3.5
  - RUBY VERSION: 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
  - INSTALLATION DIRECTORY: /usr/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/bin/ruby1.8
  - EXECUTABLE DIRECTORY: /usr/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-linux
  - GEM PATHS:
     - /usr/lib/ruby/gems/1.8
     - /home/blocke/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
     - "gem" => "--no-ri --no-rdoc"
     - :sources => ["http://gemcutter.org", "http://gems.rubyforge.org/", "http://gems.github.com"]
  - REMOTE SOURCES:
     - http://gemcutter.org
     - http://gems.rubyforge.org/
     - http://gems.github.com

According to irb, the gem path is the same except in the reverse order:

blocke:~$ irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> Gem.path
=> ["/home/blocke/.gem/ruby/1.8", "/usr/lib/ruby/gems/1.8"]

Answer 1

It doesn't work because it cannot find 'rails.rb' file (I think there is no such file in rails gem).

You can either require specific rails component, like 'activerecord':

require 'rubygems'
require 'activerecord'

Or use 'gem' command, like this:

require 'rubygems'
gem 'rails'