def get_mandatory_attribute(name)
      key = name.to_s
      raise ::ArgumentError, "unknown attribute, name=#{key}" unless self.include? key
      self[key]
    end
def get_mandatory_attribute(name)
      key = name.to_s
      raise ::ArgumentError, "unknown attribute, name=#{key}" unless self.include? key
      self[key]
    end
Gets an attribute value that has to be presented.