While creating AR extending plugin, I had some trouble to test it. Because testing a such plugin needs creating db and its absurd. So here's my solution. 

Tip #1: First, extend the AR, and name it as a MockBase. Then extend MockBase to test your class. This prevents other tests like your rails app or other plugins from failing.

Tip #2. Requiring environment.rb loads all rails env this includes AR. Thus you can you use AR in your test class.

   1  #acts_as_notifiable_test.rb
   2  begin
   3    require File.dirname(__FILE__) + '/../../../../config/environment'
   4  rescue LoadError
   5    require 'rubygems'
   6    gem 'activerecord'
   7    require 'active_record'
   8  end
   9  
  10  class MockBase < ActiveRecord::Base; end
  11  MockBase.class_eval do
  12    alias_method :save, :valid?
  13    def self.columns() @columns ||= []; end
  14  
  15    def self.column(name, sql_type = nil, default = nil, null = true)
  16      columns << ActiveRecord::ConnectionAdapters::Column.new(name.to_s, default, sql_type, null)
  17    end
  18  end
  19  
  20  class MockModel < MockBase
  21    acts_as_notifiable :callback => :after_create, :message => "hello", :recipients => "test@gmail.com"
  22  end
  23  
  24  class ActsAsNotifiableTest < Test::Unit::TestCase
  25    def test_option_message_string
  26      assert_equal "hello", MockModel.new.instance_eval{jabber_message}
  27    end

Reference

• http://snippets.dzone.com/posts/show/767
• http://nubyonrails.com/articles/the-complete-guide-to-rails-plugins-part-ii
• http://dev.rubyonrails.org/svn/rails/plugins/deadlock_retry/test/deadlock_retry_test.rb