Cannot reset variables included in URL

Question

It seems that I cannot reset a variable on a page if the variable is included in the passing URL.
For example, I am accessing page 99 using this URL:
http://hostname/pls/htmldb/f?p=101:99:13044079550212646718::::P99_INDEX:1
where P99_INDEX is an hidden item.
There is a report on page 99 with the region source:
select n, s from alpha where n=:P99_INDEX
table alpha is a test table with the entries:
SQL> select * from alpha;
N S
1 A
2 B
3 C
The page displayed the correct report if I pass in the right values for P99_INDEX (1, 2 or 3), such as
http://hostname/pls/htmldb/f?p=101:99:13044079550212646718::::P99_INDEX:1
returns
1 A
Now if I add an onLoad process on the page, which contains:
if (:P99_INDEX = 99) then
:P99_INDEX := 1;
end;
which should gives me "1 A" if I set P99_INDEX to 99:
http://hostname/pls/htmldb/f?p=101:99:13044079550212646718::::P99_INDEX:99
What I got when passing in this URL is:
No data found.
The above is a test page I created because I encountered the same issue with my current application pages. This test page confirmed what I saw.
Is this a bug or a limitation? Is there any workaround?
I had tried to create "computation" instead of "process" to reset the variable, but still did not work.
Thanks in advance.

Answer 1

Ken,
You are in the running for the "most thorough problem statement" prize. Thanks.
What you described is the intended behavior. When you set an item value in session state using the URL, that value overrides the source/default value of the item. Processes and computations that alter the item actually do get run at the specified firing points, but if they fire before item rendering, then the setting of the item with either the source/default value or the URL-passed value will take precedence.
To achieve what you want, you might try the post-computation calculation expression in the item's attributes. The PL/SQL expression that you provide here is used to adjust the value of the item just before rendering. This expression can reference items using bind variable notation and might include a call to a custom function for more complex logic. [Note: by "rendering" we mean the step where the form item is emitted as some kind of HTML, whether visible or not.]
Scott