Setting a complex default value in QGIS 3.30 that does not update automatically

Question

I'm currently using QGIS 3.30 and I'm trying to set a default value for a field based on multiple other fields, without having the default value update automatically whenever an attribute of the feature is updated. The expression I'm using is as follows:

CASE
  WHEN count("AID") = 0 THEN
    regexp_replace(
      upper(
        concat(
          coalesce(left(rpad("vorname", 1, 'X'), 3), 'X'),
          coalesce(left(rpad("name", 1, 'X'), 3), 'X'),
          coalesce(left(rpad("rolle", 1, 'X'), 3), 'X'),

          lpad('1', 4, '0'),
          @nis_project_id
        )
      ),
      '[^a-zA-Z0-9]',
      '_'
    )
  ELSE
    regexp_replace(
      upper(
        concat(
          coalesce(left(rpad("vorname", 1, 'X'), 3), 'X'),
          coalesce(left(rpad("name", 1, 'X'), 3), 'X'),
          coalesce(left(rpad("rolle", 1, 'X'), 3), 'X'),
          lpad(
            to_string(
              maximum(to_int(substr("AID", 4, 4))) + 1
            ),
            4,
            '0'
          ),
          @nis_project_id
        )
      ),
      '[^a-zA-Z0-9]',
      '_'
    )
END

The goal is to generate an ID based on the values of other fields when a new feature is created, but not to have the ID change if the feature is updated.

When I set the 'Apply default value on update' option, the ID is generated as expected, but also updates every time an attribute of the feature changes, which is not what I want.

When I do not set the 'Apply default value on update' option, the ID is not generated correctly when I input a new feature. It seems to ignore the values of other fields that are supposed to be used in generating the ID.

Could anyone guide me on how to set the default value based on the expression above, such that it considers the values of other fields at the time of feature creation and does not update automatically with each feature update?

Edit: Even with unchecked "edidable" it's still updating:

New Edit: I want the AID to be updated while the user is typing:

Here is a siplyfied test-project: Test-Project

Answer 1

You could check if the id-variable is negative (at feature creation) and modify your expression so it calculates the value for the column AID only as long as @id is negative:

( 'Apply default value on update' option activated )

 CASE WHEN @id <0 THEN
 
    CASE
      WHEN count("AID") = 0 THEN
        regexp_replace(
          upper(
            concat(
              coalesce(left(rpad("vorname", 1, 'X'), 3), 'X'),
              coalesce(left(rpad("name", 1, 'X'), 3), 'X'),
              coalesce(left(rpad("rolle", 1, 'X'), 3), 'X'),

              lpad('1', 4, '0'),
              @nis_project_id
            )
          ),
          '[^a-zA-Z0-9]',
          '_'
        )
      ELSE
        regexp_replace(
          upper(
            concat(
              coalesce(left(rpad("vorname", 1, 'X'), 3), 'X'),
              coalesce(left(rpad("name", 1, 'X'), 3), 'X'),
              coalesce(left(rpad("rolle", 1, 'X'), 3), 'X'),
              lpad(
                to_string(
                  maximum(to_int(substr("AID", 4, 4))) + 1
                ),
                4,
                '0'
              ),
              @nis_project_id
            )
          ),
          '[^a-zA-Z0-9]',
          '_'
        )
    END

ELSE "AID"

END

Answer 2

As you remarked correctly, the default-value checkbox needs to be checked, otherwise feature generation doesn't work.

If you uncheck the editable checkbox at the top of your attributes form, the ID wont change when you edit the feature.

Answer 3

Try a python function that returns a value indicating whether the record is being created or updated - something like this:

from qgis.core import *

@qgsfunction(args='auto', group='Custom')
def get_record_status(feature, parent):
    if feature.id() < 0:
       return 'Created'
    else:
       return 'Updated'

Edit the expression for the the ID default value so that it calls this function and generates the ID field if the returned value is 'Created' using the case statement you have already written. If the returned value is 'Updated' do nothing.