slightly more idiomatic, though perhaps not quite as clear as the clojure:

    def unique_keys(*hashes)
      result = {}
      hashes.each do |hash|
        result.merge! hash
      end
      return result.keys.map(&:to_s).sort.join(', ') unless result.empty?
      '<none>'
    end

https://gist.github.com/4695388