如何在Ruby on Rails 3中缓存查询

@categories = Category.joins(:posts).select(\'distinct categories.*\').order(\'label\')

   Category.cache do
      @categories = Category.joins(:posts).select(\'distinct categories.*\').order(\'label\')
    end

@categories = Rails.cache.fetch(\'categories\', :expires_in => 24.hours) { Category.joins(:posts).select(\'distinct categories.*\').order(\'label\') }

expire_fragment