def to_xml_string(str = '')
      str << "<filters #{serialized_attributes}>"
      filter_items.each {  |filter| filter.to_xml_string(str) }
      date_group_items.each { |date_group_item| date_group_item.to_xml_string(str) }
      str << '</filters>'
    end
def to_xml_string(str = '')
      str << "<filters #{serialized_attributes}>"
      filter_items.each {  |filter| filter.to_xml_string(str) }
      date_group_items.each { |date_group_item| date_group_item.to_xml_string(str) }
      str << '</filters>'
    end
Serialize the object to xml