Django foreign key on_delete: how to pass argument to callable when using models.SET

Question

This is my models.py:

def set_image(instance):
    return Image.objects.filter(user=instance.user)[0]

class User(models.Model):
    user = models.CharField(max_length=50)

class Image(models.Model):
    user = models.ForeignKey(User)
    image = models.ImageField(upload_to='media')

class Item(models.Model):
    user = models.ForeignKey(User)
    image = models.ForeignKey(
        Image,
        blank=True,
        null=True,
        on_delete=models.SET(set_image)
    )

When I delete an 'Image', it's related 'Item' calls set_image, which returns an error, because models.SET doesn't pass the instance to the callable. How can I change this behavior? Should I override models.SET or is there any other way around it?

Answer 1

Override the Image delete method to remove the Item ForeignKey

The following can be used as an intermediary model base class to add this functionality to all of your models:

from django.db import models

class Model(models.Model):
    """
    Intermediate model base class.
    """
    class Meta:
        abstract = True

    def delete(self, *args, **kwargs):
        self.clear_nullable_related()
        super(Model, self).delete(*args, **kwargs)

    def clear_nullable_related(self):
        """
        Recursively clears any nullable foreign key fields on related objects.
        Django is hard-wired for cascading deletes, which is very dangerous for
        us. This simulates ON DELETE SET NULL behavior manually.
        """
        for related in self._meta.get_all_related_objects():
            accessor = related.get_accessor_name()
            related_set = getattr(self, accessor)

            if related.field.null:
                related_set.clear()
            else:
                for related_object in related_set.all():
                    related_object.clear_nullable_related()

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