Using Model Builder "enumeration" input with multiple string values to populate field using QGIS Field Calculator [closed]

Question

In QGIS Model Builder, I’m using an "enumeration" input that allows the user to select from multiple string values (e.g., "option1", "option2", "option3").

I want to use the selected value from this enumeration input in the Field Calculator to pre-fill a field with a filename, such as "file_option1.geojson".

My goal is:

The user selects a value from the enumeration input. That value is used in a Field Calculator expression to dynamically generate a filename or label.

Questions:

1. How can I access the selected enumeration value inside the Field Calculator?
2. What is the correct syntax to use this value in an expression?
3. Is it necessary to define a variable or use another method to pass the value into the Field Calculator?

Answer 1

I have a model with an enumerate param input named enum, a vector layer param and a Field calculator. The enum param is outputting integers based on the chosen value, so I use a CASE WHEN to calculate output file name strings:

CASE
  WHEN @enum = 0 then 'out_file_1.geojson'
  WHEN @enum = 1 then 'out_file_2.geojson'
  WHEN @enum = 2 then 'out_file_3.geojson'
ELSE 'out_file_unknown.geojson'
END