How can I change the way the file shows up in the table using a mapper for the each row?

I have a form that adds a file to each id_contract with a specific file name.
I know I need to use a mapper, but I don't want a whole column to change, but only the record that I set. How can I do that?

In mapper:
Zrzut ekranu 2021-08-24 o 10.04.27

Hey @cinnamon9! I believe we resolved this over chat, but wanted to reply here as well for visibility.

We ended up adding a custom rule to the "Name of file" input:

{{!!transformer2.value ? 'already used' : ''}}

where transformer2.value runs some basic js to find all the current file names for the selected id and then check if the new value is included in those file names.
This works with each keystroke (live validation!) since transformers run automatically on input change. Since we're referring to the name input (textFile.value) in the transformer, that input is being watched so any new letter will trigger this transformer to return a true or false, depending on if the new name already exists!