def to_xml_field_uri(value)
      doc = Nokogiri::XML::Document.new
      if value.is_a?(Entry)
        doc.create_element("IndexedFieldURI", "FieldURI" => field_uri, "FieldIndex" => value.key)
      else
        doc.create_element("FieldURI", "FieldURI" => field_uri)
      end
    end
def to_xml_field_uri(value)
      doc = Nokogiri::XML::Document.new
      if value.is_a?(Entry)
        doc.create_element("IndexedFieldURI", "FieldURI" => field_uri, "FieldIndex" => value.key)
      else
        doc.create_element("FieldURI", "FieldURI" => field_uri)
      end
    end
FieldURI or IndexedFieldURI
 <t:FieldURI FieldURI="item:Sensitivity"/>
 <t:IndexedFieldURI FieldURI="contacts:EmailAddress" FieldIndex="EmailAddress1"/>

 http://msdn.microsoft.com/en-us/library/aa494315.aspx
 http://msdn.microsoft.com/en-us/library/aa581079.aspx