def rediskey idx, suffix=self.suffix
      raise RuntimeError, "No index for #{self}" if idx.to_s.empty?
      idx = Familia.join *idx if Array === idx
      idx &&= idx.to_s
      Familia.rediskey(prefix, idx, suffix)
    end
def rediskey idx, suffix=self.suffix
      raise RuntimeError, "No index for #{self}" if idx.to_s.empty?
      idx = Familia.join *idx if Array === idx
      idx &&= idx.to_s
      Familia.rediskey(prefix, idx, suffix)
    end
idx can be a value or an Array of values used to create the index.
 We don't enforce a default suffix; that's left up to the instance.
 A nil +suffix+ will not be included in the key.