anonymizer.rb

module Anonymizer
  def anonymize_attribute(attribute)
    self.send(:include, Module.new do
      define_method attribute do
        a = @attributes[attribute.to_s]
        a && attribute.to_s.ends_with?("_id") ? a.to_i : a
      end
      define_method "#{attribute}_with_anonymous" do
        send("#{attribute}_without_anonymous") unless anonymous?
      end
    end)
    alias_method_chain attribute, :anonymous
  end
  
  def anonymize_method(method, options = {})
    method_name, ending = split_method_name(method)
    self.send(:include, Module.new do
      if options[:superclass]
        define_method method do |*arguments|
          super(*arguments)
        end
      end
      define_method "#{method_name}_with_anonymous#{ending}" do |*arguments|
        send("#{method_name}_without_anonymous#{ending}", arguments) unless anonymous?
      end
    end)   
    alias_method_chain method, :anonymous 
  end

  private
  def split_method_name(method)
    if ['?', '!'].include?(method.to_s.last)
      [method.to_s[0..-2], method.to_s.last]
    else
      [method, nil]
    end
  end
end

post.rb

# == Schema Information
#
# Table name: posts
#
#  id                  :integer(11)     not null, primary key
#  user_id             :integer(11)
#  anonymous           :boolean(1)
#  ...
class Post < ActiveRecord::Base
  extend Anonymizer

  belongs_to :user
  anonymize_method :user
  anonymize_attribute :user_id
end

usage_example.rb

>> p = Post.find(:first, :conditions => {:anonymous => true})
=> #<Post id: 31, ... >
>> p.user
=> nil
>> p.user_id_with_anonymous
=> nil
>> p.user_id_without_anonymous
=> 1234
>> p.user_without_anonymous
=> #<User id: 1234, ... >