A little known gotcha, the cause of which can be obscure… You can oftentimes see suggested, even when discussing best practices, snippets like the following:

$LOAD_PATH.unshift(File.dirname(__FILE__) + '/../lib') unless \
  $LOAD_PATH.include?(File.dirname(__FILE__) + '/../lib')

If you intend to use Ruby’s [autoload], then do not do that. If you do you’ll run into this issue.

Rather, if you have to keep backward compatibility, insert File.expand_path, or use some of methods from the Pathname standard library. The Pathname has the advantage of being more declarative. But you should take care to not feed ‘~’ Pathname, or just move on to Ruby 1.9.2….

Example (incl. 1.8.7):

require 'pathname'
lp = Pathname.new(__FILE__).dirname.realpath.parent + 'lib'
$LOAD_PATH.unshift(lp) unless $LOAD_PATH.include?(lp)

If you don’t have to worry about 1.8.7:

Example (1.9.2):

require_relative '../lib/mylibrary'