XQuery if exists conditional insert / replace - sql-server-2008

Question

What would the XQuery look like to check if a node exists, and if it does then run a replace statement, if not then an insert statement?
Here's what I have in mind. I want to store whether or not a user has read an important message in XML. Here's what the data would look like.
<usersettings>
<message haveRead="0" messageId="23" ></message>
<message haveRead="1" messageId="22" ></message>
</usersettings>
Basically this XML tells me that the user has read one message, while the other message still needs to be viewed / read.
I want to combine my insert / replace xquery into one statement. Here's what I had in mind.
UPDATE WebUsers SET UserSettings.modify('
declare default element namespace "http://www.test.com/test";
IF a node exists with the messageId
code to replace node with new update
ELSE
code to insert a new node with the provided variables
')
WHERE Id = #WebUserId

Answer 1

I haven't found a really satisfactory way of doing this, but one of these might work for you. I like the first technique better, but I'm annoyed that I haven't found a more elegant way of doing it.
Make sure the node always exists first
DECLARE #messageID int;
SET #messageID=24;
DECLARE #myDoc xml;
SET #myDoc =
'<usersettings>
<message haveRead="0" messageId="23" >msg</message>
<message haveRead="1" messageId="22" >msg</message>
</usersettings>';
SELECT #myDoc;
SET #myDoc.modify('
insert
if (count(//message[#messageId=sql:variable("#messageID")]) = 0)
then <message haveRead="0">new msg</message>
else()
as last into (/usersettings)[1]
');
SELECT #myDoc;
--now do the rest, safe that the node exists
Switching
DECLARE #myDoc xml;
SET #myDoc =
'<usersettings>
<message haveRead="0" messageId="23" >msg</message>
<message haveRead="1" messageId="22" >msg</message>
</usersettings>';
SELECT #myDoc;
DECLARE #messageID int;
SET #messageID=23;
IF #myDoc.exist('//message[#messageId=sql:variable("#messageID")]') = 1
BEGIN
SET #myDoc.modify('replace value of (//message[#messageId=sql:variable("#messageID")]/text())[1]
with "test"')
END
ELSE
BEGIN
SET #myDoc.modify('insert <message haveRead="0">new msg</message>
into (/usersettings)[1]')
END
SELECT #myDoc;