You can find at runtime where a method was defined. This is a little hack to then open that method in textmate

open_method_def_in_textmate.rb

> m = 2.days.method(:ago)
 => #<Method: Fixnum(ActiveSupport::CoreExtensions::Numeric::Time)#ago> 
> m.__file__
 => "/Users/paul/.rvm/gems/ree-1.8.7-2011.03@deals/gems/activesupport-2.3.5/lib/active_support/core_ext/numeric/time.rb" 
> m.__line__
 => 64 
> class Method; def open_in_textmate; `mate -l #{__line__} #{__file__}`; end; end
  => nil 
> 2.days.method(:ago).open_in_textmate
  => ""