SET_NULL on deleting erases the relation

[FranciscodeMaussion]

Problem Statement

We were having trouble un-deleting objects that had a relation to a model with on_delete=models.SET_NULL since the relation is lost and couldn't be automatically restored when undeleting.

For example:

class ModelB(SafeDeleteModel):
    pass

class ModelA(SafeDeleteModel):
    model_b = models.OneToOneField(
        ModelB,
        on_delete=models.SET_NULL,
        null=True,
        blank=True,
    )

Solution/or maybe a Workaround

I created a custom policy SET_NULL_ONLY_ON_HARD_DELETE

def SET_NULL_ONLY_ON_HARD_DELETE(collector, field, sub_objs, using):
    """
    This method is used to set the field to null only when related object is hard deleted.

    As for soft_delete_cascading deletion this works as SET_NULL/SET/SET_DEFAULT.
    See https://github.com/makinacorpus/django-safedelete/blob/master/safedelete/models.py#L247.

    Default collector(used on hard delete) does not have the edges attribute,
    so we can use it to differentiate between hard and soft delete.
    Default collector: https://github.com/django/django/blob/stable/5.1.x/django/db/models/deletion.py#L94
    Admin/Soft delete collector: https://github.com/django/django/blob/stable/5.1.x/django/contrib/admin/utils.py#L186
    """
    if hasattr(collector, "edges"):
        return models.DO_NOTHING(collector, field, sub_objs, using)
    return models.SET_NULL(collector, field, sub_objs, using)


SET_NULL_ONLY_ON_HARD_DELETE.lazy_sub_objs = True

I don't know if should be contributed to the main package. Just let me know!