Model class fields for handling foreign key deletions

models.py

...

severity_level = models.ForeignKey(
        'business.SeverityLevel', null=True, blank=True,
        on_delete=models.SET(
            None
        )
    )
    status = models.ForeignKey(
        'CaseStatus', null=True, blank=True,
        on_delete=models.SET(
            get_default_status
        )
    )
    
...