The difference between extend and include in Ruby

ruby_extend_and_include.rb

require 'test/unit'
require 'active_support/all'

class TestExtendAndInclude < Test::Unit::TestCase

   module SomethingExtended
      mattr_accessor :data_attributes

      def data_attribute(name)           
         self.data_attributes ||= []
         self.data_attributes << name
         attr_accessor name
      end
   end

   module SomethingIncluded
      def self.included(base)
         base.class_attribute :data_attributes
         base.data_attributes = []
         base.extend(ClassMethods)
      end

      module ClassMethods
         def data_attribute(name)
            self.data_attributes << name  
            attr_accessor name
         end
      end
   end

   class StuffExtendsSomething
      extend SomethingExtended
      data_attribute :message
   end

   class StuffIncludesSomething
      include SomethingIncluded
      data_attribute :message
   end

   
   def setup
      @x = StuffExtendsSomething.new
      @y = StuffIncludesSomething.new
      @x.message = "hello"
      @y.message = "hello"
   end

   def test_that_they_both_define_the_same_attribute
      assert_equal(@x.message,@y.message,
      "both classes should have the same value for :message")
   end

   def test_that_they_both_have_the_same_class_attributes
      assert_equal(@x.class.data_attributes,@y.class.data_attributes,
      "both classes should have the same data attributes")
   end

end