# File lib/couchrest/model/persistence.rb, line 155
        def unique_id(method = nil, &block)
          if method
            define_method :set_unique_id do
              self['_id'] ||= self.send(method)
            end
          elsif block
            define_method :set_unique_id do
              uniqid = block.call(self)
              raise ArgumentError, "unique_id block must not return nil" if uniqid.nil?
              self['_id'] ||= uniqid
            end
          end
        end