Why does this throw an error?

Question

just trying to modify an element value here...
declare
v_xml_new xmltype;
doc dbms_xmldom.DOMDocument;
ndoc dbms_xmldom.DOMNode;
node dbms_xmldom.DOMNode;
childnode dbms_xmldom.DOMNode;
nodelist dbms_xmldom.DOMNodelist;
docelem dbms_xmldom.DOMElement;
buffer ta_debug_log.message%type;
begin
v_xml_return.schemaValidate;
ta_debug.writeDebug('DEBUG: xml instance validated '||C_PROCEDURE_NAME||'.');
exception
when others then
-- failed validation
ta_system.writeSystem('ERROR: xml instance message failed schema validation '||C_PROCEDURE_NAME||'.');
-- return a new wellformed instance
ta_debug.writeDebug('DEBUG: creating new empty instance '||C_PROCEDURE_NAME||'.');
v_xml_new := xmltype.createXML(xmldata => rm_xml_survey_maintenance.C_EMPTY_SURVEY_XML_INST,
schema => rm_xml_survey_maintenance.C_SURVEY_SCHEMA_NAME);
ta_debug.writeDebug('DEBUG: creating dom reference '||C_PROCEDURE_NAME||'.');
doc := dbms_xmldom.newDOMDocument(v_xml_new);
-- attempt to populate return xml instance with source info
ta_debug.writeDebug('DEBUG: attempting to acquire and populate source info '||C_PROCEDURE_NAME||'.');
ta_debug.writeDebug('DEBUG: source information requesterID is '||
ta_xml_utils.xml_extract_no_exception(v_xml_return, '/SurveyRequest/messageSource/requesterID/text()')||' '||C_PROCEDURE_NAME||'.');
docelem := dbms_xmldom.getDocumentElement(doc);
nodelist := dbms_xmldom.getElementsByTagName(docelem, 'requesterID');
node := dbms_xmldom.item(nodelist, 0);
childnode := dbms_xmldom.getFirstChild(node);
the last line causes the code the throw:
ERROR: SQLCODE = -31020 SQLERRM = ORA-31020: The operation is not allowed,
Reason: Invalid pl/sql DOM Node hdl
ORA-31154: invalid XML
(rm_xml_survey_maintenance.create_cad_plan_pair).
this code is operating after an xmltype fails to validate. would this be the cause?
are there any simpler methods using pl/sql, the xmltype and xpath to modify contents of an xmltype instance?
any advice most appreciated.

Answer 1

I can't answer the better/simpler method part of the question, other than to say there is an UpdateXML function but it works only from within the scope of a SQL statement. Not sure whether you'd want to make a SQL call just to do something to your XMLType that exists only in PL/SQL.
In regard to your current error, I can only say the following script does not produce an error. If you can develop a test case/script that reproduces the error that would be helpful. You may even uncover the error at that point. Have you confirmed the contents of v_xml_new and if on 10g used DBMS_UTILITY.Format_Error_Backtrace to verify the line that threw the error?
declare
   v_xml_new xmltype;
   doc       dbms_xmldom.domdocument;
   nodelist  dbms_xmldom.domnodelist;
   node      dbms_xmldom.domnode;
   childnode dbms_xmldom.domnode;
begin
  v_xml_new := XMLTYPE(
  '<?xml version="1.0" encoding="UTF-8"?>' ||
  '<SurveyRequest>' ||
   '<messageSource>' ||
    '<requesterID>U571</requesterID>' ||
   '</messageSource>' ||
  '</SurveyRequest>'
  doc := dbms_xmldom.newdomdocument(v_xml_new);
  nodelist  := dbms_xmldom.getelementsbytagname(doc, 'requesterID');
  node      := dbms_xmldom.item(nodelist, 0);
  childnode := dbms_xmldom.getfirstchild(node);
end;