Question

I have a general question about designing Django models and considering the effect that has on a cascade delete. Suppose the following simple design of an Asset Management app.

class Asset(models.Model):
    aquire_date = models.DateField()
    cost = models.DecimalField(max_digits=16, decimal_places=2)
    description = models.CharField(max_length=30)
    account = models.ForeignKey(Account)
    vendor = models.ForeignKey(Vendor)
    department = models.ForeignKey(Department)

class Vendor(models.Model):
    name = models.CharField(max_length=70)
    city = models.CharField(max_length=50
    phone = PhoneNumberField()
    email = models.EmailField()

class Account(models.Model):
    account_number = models.IntegerField()
    description = models.CharField(max_length=50)

class Department(models.Model):
    number = models.IntegerField(unique=True)
    name = models.CharField(max_length=50)

So each Asset has 3 ForeignKey fields to other tables. Reading the Django documentation it says ‘any objects which had foreign keys pointing at the object to be deleted will be deleted along with it’. I read that to mean if I delete a Department object, the Asset object, or objects, the ForeignKey is referring to will be deleted as well. But if I delete an Asset, the Department, Vendor, and Account remains.

Is that the correct way to understand cascade deletes in Django?

What I would prefer is

• when an Asset is deleted, no department, vendor, or account is deleted;
• when a Department is deleted, no Asset is deleted, but all ForeignKey fields that point to that Department are set to null.

Is that possible?

Answer 1

Yes, your understanding is correct.

Solution: When creating a model ForeignKey field, supply on_delete=django.db.models.SET_NULL. You need to combine this with null=True to allow this because the default is False.