Dynamically reference :old.column_ name in trigger

Question

Hi all,
I am writing an application for which i would like to include a limited audit trail.
I have a form that is based on a table with approx 100 columns and wish to write the :old.column_name value on fields that have been updated, along with userid and date/time to an "audit" table.
I think that I can do this by using the Before_update trigger and then the IF UPDATING ('column name') condition. However, for maintenance and readabiility, I wish to be able to dynamically reference the :old.column_name value, which I cannot get working.
I know it can be done with a large IF statement, but due to the number of columns involved, this can lead to a maintenance nightmare. Also, if I can get this working, I want to include the procedure in a library for other applications, thus the need for dynamically referencing the table and column names.
My proposed logic for the before_update trigger is:
Declare Vcolumn_name Varchar2(50)
Get column names for table (using select cursor)
loop through cursor
Vcolumn_name := column name
IF UPDATING (Vcolumn_name) then
insert userid, timestamp, :old.Vcolumn_name into audit table
end IF
next cursor row
:old.Vcolumn_name is where I am having trouble.
Is this possible?
Thanks
Peter

Answer 1

The pre-update trigger won't store the value of the old.column . If you are querying the data and then updating then use the POST-QUERY trigger to store the current values in global variables. In the PRE-UPDATE trigger assign the global variables to the columns of the audit tables.
For instance in the PRE-FORM trigger initialize the global variables :
:GLOBAL.VAR1 := ' ';
:GLOBAL.VAR2 := ' ';
In the POST-QUERY trigger
:GLOBAL.VAR1 := :BLOCK.COLUMN1;
:GLOBAL.VAR2 := :BLOCK.COLUMN2;
In the PRE-UPDATE trigger
INSERT INTO AUDIT_TABLE (COL1, COL2)
VALUES(:GLOBAL.VAR1, :GLOBAL.VAR2);
Hope this helps.