everything.rb

module Helpers
  def self.constantize(camel_cased_word)
    names = camel_cased_word.split('::')
    names.shift if names.empty? || names.first.empty?

    constant = Object
    names.each do |name|
      constant = constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name)
    end
    constant
  end
end

class String
  def constantize
    Helpers.constantize(self)
  end
end

module HasProviderSubclass
  module ClassMethods
    def type_to_class_string type
      @type_to_class_string ||= Hash.new do |h, type|
        h[type] = "#{self.to_s.split('::').first}::#{type.capitalize}"
      end
      @type_to_class_string[type]
    end

    def type_to_class type
      @type_to_class ||= Hash.new { |h, cs| h[cs] = type_to_class_string(cs).constantize }
      @type_to_class[type]
    end

    private

    def __setup_provider_variables
      @type_to_class_string = Hash.new do |h, type|
        h[type] = "#{self.to_s.split('::').first}::#{type.capitalize}"
      end
      @type_to_class = Hash.new { |h, cs| h[cs] = type_to_class_string(cs).constantize }
    end


  end

  module InstanceMethods
  end

  def self.included(receiver)
    receiver.extend         ClassMethods
    receiver.send :include, InstanceMethods
  end
end

class UserIdentifier
  include HasProviderSubclass
  class Facebook < UserIdentifier
  end
end

class ExternalContactList
  include HasProviderSubclass
  class Facebook < ExternalContactList
  end
end

# Intended Behavior:
puts ExternalContactList.type_to_class('facebook') # => ExternalContactList::Facebook
puts UserIdentifier.type_to_class('facebook')      # => UserIdentifier::Facebook
puts UserIdentifier::Facebook.type_to_class('facebook') # => UserIdentifier::Facebook

# Actual Behavior
puts ExternalContactList.type_to_class('facebook') # => UserIdentifier::Facebook
puts UserIdentifier.type_to_class('facebook')      # => UserIdentifier::Facebook
puts UserIdentifier::Facebook.type_to_class('facebook') # => UserIdentifier::Facebook