example.rb

require 'bundler/setup'
Bundler.require

DATABASE = './test.sqlite3'

# Setup

File.unlink(DATABASE) if File.exist?(DATABASE)

ActiveRecord::Base.establish_connection(
  adapter:  'sqlite3',
  database: DATABASE
)

class InitializeDatabase < ActiveRecord::Migration
  def change
    create_table :coffees do |t|
      t.string :size
    end
  end
end

InitializeDatabase.migrate(:up)

# Definition

class Coffee < ActiveRecord::Base
  extend Enumerize

  enumerize :size, in: %w(short middle large)
end

# Assertion

require 'test/unit'

class EnumerizeTest < Test::Unit::TestCase
  def setup
    Coffee.destroy_all
  end

  def test_create
    Coffee.create(size: 'short')

    assert_equal 1, Coffee.count
    assert_equal 'short', Coffee.first.size # This assertion is failed!
  end
end

Gemfile

source 'https://rubygems.org'

gem 'activerecord', '4.0.0.beta1', require: 'active_record'
gem 'enumerize', github: 'brainspec/enumerize'

gem 'sqlite3'

Gemfile.lock

GIT
  remote: git://github.com/brainspec/enumerize.git
  revision: d07fda637ae07dcb17b37e0128767aa2eebbf9b3
  specs:
    enumerize (0.5.1)
      activesupport (>= 3.2)

GEM
  remote: https://rubygems.org/
  specs:
    activemodel (4.0.0.beta1)
      activesupport (= 4.0.0.beta1)
      builder (~> 3.1.0)
    activerecord (4.0.0.beta1)
      activemodel (= 4.0.0.beta1)
      activerecord-deprecated_finders (~> 0.0.3)
      activesupport (= 4.0.0.beta1)
      arel (~> 4.0.0.beta1)
    activerecord-deprecated_finders (0.0.3)
    activesupport (4.0.0.beta1)
      i18n (~> 0.6.2)
      minitest (~> 4.2)
      multi_json (~> 1.3)
      thread_safe (~> 0.1)
      tzinfo (~> 0.3.33)
    arel (4.0.0.beta1)
    atomic (1.0.1)
    builder (3.1.4)
    i18n (0.6.2)
    minitest (4.6.1)
    multi_json (1.6.1)
    sqlite3 (1.3.7)
    thread_safe (0.1.0)
      atomic
    tzinfo (0.3.35)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord (= 4.0.0.beta1)
  enumerize!
  sqlite3

USAGE

$ bundle exec ruby example.rb

This is my result.

Run options: 

# Running tests:

[1/1] EnumerizeTest#test_create = 0.01 s
  1) Failure:
test_create(EnumerizeTest) [example.rb:46]:
<"short"> expected but was
<nil>.

Finished tests in 0.032182s, 31.0733 tests/s, 62.1465 assertions/s.
1 tests, 2 assertions, 1 failures, 0 errors, 0 skips

ruby -v: ruby 2.0.0p0 (2013-02-24 revision 39473) [x86_64-darwin11.4.2]