hstore_rollback.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.0.2'
    gem 'arel'
    gem 'pg'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
#require 'minitest/autorun'
require 'logger'
# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: 'rails_test')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.string :hstore_test
  end
end

class HstoreTest < ActiveRecord::Migration
  def change
    change_table :users do |t|
      t.hstore :hstore_test, null: false, default: {}
    end
  end
end

HstoreTest.new.change

# -- create_table(:users)
# D, [2013-12-29T15:08:58.175459 #29519] DEBUG -- :    (467.3ms)  CREATE TABLE "users" ("id" serial primary key, "hstore_test" character varying(255)) 
#    -> 0.4766s
# -- change_table(:users)
# D, [2013-12-29T15:08:58.177064 #29519] DEBUG -- :    (0.8ms)  ALTER TABLE "users" ADD COLUMN "hstore_test" hstore DEFAULT '' NOT NULL
# E, [2013-12-29T15:08:58.177194 #29519] ERROR -- : PG::UndefinedObject: ERROR:  type "hstore" does not exist
# LINE 1: ALTER TABLE "users" ADD COLUMN "hstore_test" hstore DEFAULT ...

Tested on Rails 4.0.2