all_cache_key.rb

module AllCacheKey
  extend ActiveSupport::Concern

  module ClassMethods
    def cache_key
      pluck("COUNT(*)", "MAX(updated_at)").flatten.map(&:to_i).join("-")
    end
  end
end

Note the multi-valued pluck is a Rails 4 feature, Rails 3 will need something slightly different

This is similar to Caching with Rails guide cache_key_for_products method, isn't it ?
Or i'ts more optimized ?

For Rails 3 you need a little bit more code:

def cache_key
  sql = select('COUNT(*), MAX(updated_at)').to_sql
  connection.execute(sql).to_a.flatten.map(&:to_i).join('-')
end