Help with not updating user id

[cortina]

Hi all!
I use TM1 9.5.2 and have the following task.
In measures dimension I have element 'UserID' that is not empty when user enters transaction code and transaction sum. This simply rule

['UserID'] =S: IF(['Code']>0 & ['Sum']>0, ATTRS('}Clients', TM1User(), '}TM1_DefaultDisplayValue'),' ');

doesn't work properly. As you can see when new user logs in the system the field 'UserID' has this user's id despite the fact whether the user edited something.

Please, help with writing a rule that would display last person who edited the transaction. Or if it is complicated the first person who entered transaction code and transaction sum.

Thanks in advance!