Form field with dynamic id validation

so i could be wrong but i think it could be something like this - say if you are doing it in a form validation array,
and somehow you get the id in there - which keep in mind could be in a hidden form field

PHP Code:

           array(
               $fieldname = 'mechanic_' . $theid ; 
                'field' =>  $fieldname,
                'label' => 'Mechanic',
                'rules' => 'trim|required|max_length[100]'
            ), 


also keep in mind that you can do form validation in separate methods - so for example you could check that the ID is valid before validating the mechanic field, etc