gistfile1.rb

# class to test
require 'rubygems'
require 'active_record'

class Post < ActiveRecord::Base
  validates_presence_of :title

  before_save :create_slug

  private
    def create_slug
      self.slug ||= self.title.gsub(' ','-').downcase
    end
end


# needed AR testing environment: I would not mock the entire connection, columns calls, etc,
# If that's your choice see: http://blog.jayfields.com/2006/12/rails-activerecord-unit-testing.html

require 'sqlite3'
ActiveRecord::Base.establish_connection(
  :adapter => 'sqlite3',
  :database  => 'test.db'
)

ActiveRecord::Schema.define do
  create_table :posts, :force => true do |t|
    t.column :title, :string
    t.column :slug, :string
  end
end


# the tests properly said:

require 'test/unit'
require 'mocha'

class PostTest < Test::Unit::TestCase

  def setup
    @post = Post.new :title => "First post"
  end

  def test_slug_generated_on_creation
    @post.expects(:create).returns true     # expects(:save) doesn't execute before_save callback,
                                            # so we need to mock/stub the underlying create/update call
    @post.save
    assert_equal 'first-post', @post.slug
  end
  
  def test_slug_doesnt_change_on_updates
    @post.save
    @post.title = 'another title'
    @post.expects(:update).returns true     # expects(:save) doesn't execute before_save callback,
                                            # so we need to mock/stub the underlying create/update call
    @post.save
    assert_equal 'first-post', @post.slug
  end
  
end