def attributes
      attrs = Hash.new
      attribute_list.each do |attribute|
        value = self.send(attribute)
        attrs[attribute] = value unless value.nil?
      end
      attrs
    end
def attributes
      attrs = Hash.new
      attribute_list.each do |attribute|
        value = self.send(attribute)
        attrs[attribute] = value unless value.nil?
      end
      attrs
    end
A hash of all non-nil attributes of this object.
 Keys of this hash are <tt>:id</tt>, <tt>:user</tt>,
 and <tt>:timestamp</tt>. For a Node also <tt>:lon</tt>
 and <tt>:lat</tt>.

 @return [Hash] a hash of all non-nil attributes of this object.