Prevent delete in Django model

Question

I have a setup like this (simplified for this question):

class Employee(models.Model):
    name = models.CharField(name, unique=True)

class Project(models.M         


        

Answer 1

This would wrap up solution from the implementation in my app. Some code is form LWN's answer.

There are 4 situations that your data get deleted:

• SQL query
• Calling delete() on Model instance: project.delete()
• Calling delete() on QuerySet innstance: Project.objects.all().delete()
• Deleted by ForeignKey field on other Model

While there is nothing much you can do with the first case, the other three can be fine grained controlled. One advise is that, in most case, you should never delete the data itself, because those data reflect the history and usage of our application. Setting on active Boolean field is prefered instead.

To prevent delete() on Model instance, subclass delete() in your Model declaration:

    def delete(self):
        self.active = False
        self.save(update_fields=('active',))

While delete() on QuerySet instance needs a little setup with a custom object manager as in LWN's answer.

Wrap this up to a reusable implementation:

class ActiveQuerySet(models.QuerySet):
    def delete(self):
        self.save(update_fields=('active',))


class ActiveManager(models.Manager):
    def active(self):
        return self.model.objects.filter(active=True)

    def get_queryset(self):
        return ActiveQuerySet(self.model, using=self._db)


class ActiveModel(models.Model):
    """ Use `active` state of model instead of delete it
    """
    active = models.BooleanField(default=True, editable=False)
    class Meta:
        abstract = True

    def delete(self):
        self.active = False
        self.save()

    objects = ActiveManager()

Usage, just subclass ActiveModel class:

class Project(ActiveModel):
    ...

Still our object can still be deleted if any one of its ForeignKey fields get deleted:

class Employee(models.Model):
    name = models.CharField(name, unique=True)

class Project(models.Model):
    name = models.CharField(name, unique=True)
    manager = purchaser = models.ForeignKey(
        Employee, related_name='project_as_manager')

>>> manager.delete() # this would cause `project` deleted as well

This can be prevented by adding on_delete argument of Model field:

class Project(models.Model):
    name = models.CharField(name, unique=True)
    manager = purchaser = models.ForeignKey(
        Employee, related_name='project_as_manager',
        on_delete=models.PROTECT)

Default of on_delete is CASCADE which will cause your instance deleted, by using PROTECT instead which will raise a ProtectedError (a subclass of IntegrityError). Another purpose of this is that the ForeignKey of data should be kept as a reference.