built_value generator for specific directory

Question

I am using source_gen library to generate built_value files for specific directory (generated). source_gen is working well with isar to generate files to seperate directory but it is failing for built_value for some reason.

Could you help me how to specify output gen directory in built_value so that my models directory is not overloaded with .g.dart files?

Here is the build.yaml file:

targets:
  $default:
    builders:
      # A SharedPartBuilder which uses the combining builder
      source_gen:combining_builder:
        options:
          build_extensions:
            '^lib/{{}}.dart': 'lib/generated/{{}}.g.dart'

      # A PartBuilder or LibraryBuilder
      built_value_generator:built_value:
        options:
          build_extensions:
            '^lib/{{}}.dart': 'lib/generated/{{}}.g.dart'

Here is the error message in output:

[SEVERE] built_value_generator:built_value on lib/core/model/response/work_session.dart:
Error in BuiltValueGenerator for abstract class WorkSession extends SoterApiResponse implements Built<WorkSession, dynamic>.
Please make the following changes to use BuiltValue:

1. Import generated part: part 'work_session.g.dart';

I am sure I gave correct directive in work_session file:

part '../../../generated/core/model/response/work_session.g.dart';