database_cleaner transactionの設定 ref: http://qiita.com/yoshitsugu@github/items/3470dbcadfdd677be543

spec_helper.rb

Spork.prefork do
  ...
  RSpec.configure do |config|
   ....
    # for database_clener
    config.before(:suite) do
      DatabaseCleaner.clean_with(:truncation,{:except => %w{except_tables}})
      DatabaseCleaner.strategy = :transaction
    end

    config.before(:each) do
      DatabaseCleaner.start
    end

    config.after(:each) do
      DatabaseCleaner.clean
    end
  end
end