Question

We recently had a problem where, after a series of commits had occurred, a backend process failed to run. Now, we were good little boys and girls and ran rake test after every check-in but, due to some oddities in Rails’ library loading, it only occurred when we ran it directly from Mongrel in production mode.

I tracked the bug down and it was due to a new Rails gem overwriting a method in the String class in a way that broke one narrow use in the runtime Rails code.

Anyway, long story short, is there a way, at runtime, to ask Ruby where a method has been defined? Something like whereami( :foo ) that returns /path/to/some/file.rb line #45? In this case, telling me that it was defined in class String would be unhelpful, because it was overloaded by some library.

I cannot guarantee the source lives in my project, so grepping for 'def foo' won’t necessarily give me what I need, not to mention if I have many def foo‘s, sometimes I don’t know until runtime which one I may be using.

Answer 1

This is really late, but here’s how you can find where a method is defined:

http://gist.github.com/76951

# How to find out where a method comes from. # Learned this from Dave Thomas while teaching Advanced Ruby Studio # Makes the case for separating method definitions into # modules, especially when enhancing built-in classes. module Perpetrator   def crime   end end  class Fixnum   include Perpetrator end  p 2.method(:crime) # The '2' here is an instance of Fixnum. #<Method: Fixnum(Perpetrator)#crime> 

If you’re on Ruby 1.9+, you can use source_location

require 'csv'  p CSV.new('string').method(:flock) # => #<Method: CSV#flock>  CSV.new('string').method(:flock).source_location # => ['/path/to/ruby/1.9.2-p290/lib/ruby/1.9.1/forwardable.rb', 180] 

Note that this won’t work on everything, like native compiled code. The Method class has some neat functions, too, like Method#owner which returns the file where the method is defined.

EDIT: Also see the __file__ and __line__ and notes for REE in the other answer, they’re handy too. — wg