def define_hash_method
      keys = @keys
      define_method(:hash) do
        keys.map { |key| send(key) }.push(self.class).hash
      end
    end
def define_hash_method
      keys = @keys
      define_method(:hash) do
        keys.map { |key| send(key) }.push(self.class).hash
      end
    end
Define a #hash method based on the instance's values identified by #keys

 @return [undefined]

 @api private