Trouble hiding / showing element with DA based on results of query

Question

Hi all,
I have a field that I would like to valid either on keypress or lose focus. What I need to do is query a table with the value entered to determine if it exists. If it does, I want to show a little green checkmark, if not a red 'x'. I figure that I can use getElementById() to hide or show the image, but I can't seem to figure out how to do that based on the result of the query.
I've tried this so far:
DA
event: lose focus
type: item
item: P3_PO_REF_NUMBER
condition: not null
true action: execute pl/sql code
code:
BEGIN
declare
l_exists number;
begin
  select count(*) into l_exists
  from PO_LINE
  where BUSINESS_UNIT = 'METRO' and
    (PO_ID = to_char(:P3_PO_REF_NUMBER,'FM000000000') or PO_ID = 'S'||:P3_PO_REF_NUMBER);
  if l_exists > 0 then
    apex_util.set_session_state('P3_VALID', 'VALID');
  else
    apex_util.set_session_state('P3_VALID', 'NOT_VALUE');
  end if;
end;
END;page items to submit: P3_PO_REF_NUMBER
page items to return: P3_VALID
Eventually I thought that I'd create another hide/show DA based on the value of P3_VALID, but before I went much further, I wanted to check to see how this worked. It didn't. Error: exception thrown and not caught.
Any thoughts as to what I'm missing, or am I doing this completely wrong?
Thanks,
Joe

Answer 1

Ok, just for closure. I did get this working.
I ended up deleting the original P3_PO_REF_NUMBER item and recreated it, and everything began to work. As I was doing this, I noticed that it seemed like things would begin to go awry when I changed the item's name, even if I changed it back again. It was almost as if there was an associated between the DA that became corrupted once the item name was changed. If I took the item out of the page items to submit field, no error. Of course the DA wouldn't fire correctly then either because it wasn't getting any value changes, but when I added the item back into the page items to submit field, I go an error.
So:
When I created the second PO_REF_NUMBER item, I did not delete the original first, so the new one of course had to be named differently. Then I modified the DA to use the new item name (PO_REF_NUM). It worked! Cool. So, no wanting to get rid of the original yet, I rename it, and then renamed the second one to what the original was (I have other code using that item name). Then I changed the DA again...but now it failed - unless I blanked out the page items to submit field...
3rd time is a charm. So, this time, I made sure that no items were named "PO_REF_NUMBER", then I created the PO_REF_NUMBER again, naming it exactly as I needed it to be so I wouldn't have to change it later. Then I set out to tweak the field (size, etc), but did not touch the name. Now my DA is working nicely! It sets a hidden field's value, then another DA shows or hides an icon based on that hidden field's.
Hopefully someone will find this useful,
Joe