Modifying on_delete from default CASCADE to SET_NULL and running `aerich migrate` does not generate migration file

[dlee1579]

Describe the bug
I am attempting to change a foreign key on my data model to explicity set on_delete behavior to SET_NULL. Since default on_delete behavior is CASCADE, after running aerich migrate I wouild expect a new migration file to have been generated. Instead, no changes are recognized.

To Reproduce
change

example_fk_field = fields.ForeignKeyField("models.OtherModel")

to

example_fk_field = fields.ForeignKeyField("models.OtherModel", on_delete=fields.SET_NULL)

Expected behavior
Database migration file should be generated upon running aerich migrate.

Additional context
None

[waketzheng]

I tried django with the following steps:

mkdir myproject
cd myproject
python3.12 -m venv .venv
source .venv/bin/activate
uv pip install django
python -m django startproject $(basename `pwd`) .
python manage.py startapp polls
echo "INSTALLED_APPS += ['polls']" >> myproject/settings.py
echo 'from django.db.models import CASCADE, Model, CharField, Fo
reignKey
class Group(Model):
    name = CharField(max_length=20)
class Role(Model):
    name = CharField(max_length=20)
    group = ForeignKey(Group, on_delete=CASCADE, null=True)
' > polls/models.py
python manage.py makemigrations polls
python manage.py migrate polls
python -c "f='polls/models.py';a='CASCADE';b='SET_NULL';from pathlib import Path;p=Path(f);p.write_bytes(p.read_bytes().replace(a.encode(),b.encode()))"
python manage.py makemigrations polls
python manage.py sqlmigrate polls 0002

Output is:

BEGIN;
--
-- Alter field group on role
--
-- (no-op)
COMMIT;

That is to say django does not generate SQL when modifying on_delete from default CASCADE to SET_NULL.

May be we should handle the on_delete programmatically and remove the constraint when generating such fields.