"Uninitialized constant" error when including a module

I am trying to reference an association extension but it errors with:

NameError (uninitialized constant User::ListerExtension):
  app/models/user.rb:2:in `<class:User>'

Here is my implementation:

app/models/user.rb

class User < ActiveRecord::Base
  include ListerExtension
  
  has_and_belongs_to_many :roles, :uniq => true, :extend => Lister

lib/lister.rb

module ListerExtension
  def lister
    self.map(&:to_s).join(', ')
  end
end

I am using Rails v3.1.3.

Andrew Marshall has an excellent point about the auto-load setup (see the question he links for more on that), but also: Because you named your class ListerExtension, Rails will be looking for a file named lister_extension.rb - not lister.rb. It's smart, but it's not that smart.

Hope that helps!