Random Key From Ruby Hash

Returns a random key from your hash, but won't repeat until it's done every one.  (I was using this to help quiz myself.)

class Hash
  @keys_used = []

  def random_key
    @keys_used = [] if @keys_used.size == self.size
    key = self.keys[rand(self.size)]
    while @keys_used.include?(key)
      key = self.keys[rand(self.size)]
    end
    @keys_used << key
    return key
  end
end