Django on_delete = models.SET()

Question

Below is my table :

class Project(models.Model):
    name = models.CharField(max_length=100, null=False)
    desc = models.CharField(max_length=100, null=False)

    def __str__(self):
        return self.name

class Activity(models.Model):
    title = models.CharField(max_length=100, null=False)
    project = models.ForeignKey(Project, on_delete=models.SET(some_func), null=False, 
              related_name='project_model')

So above is my table and when one of the datas of "project" table gets deleted then i want to assign "id or name" of the deleted "project" data to foreign key of the "activity" table. How can i achieve this? pls help.

Navid Zarepak · Accepted Answer · 2020-07-17 10:04:01Z

1

I assume you're doing this for logging purposes. I don't think using a ForeignKey field is a good option for storing an id that is not there anymore since the expected behavior of a ForeignKey is usually different than this.

But if you really need the id to be stored, I recommend using an IntegerField instead.

And to create a new Activity instance whenever a Project instance is deleted:

Overriding delete method:

class Project(models.Model):
    name = models.CharField(max_length=100, null=False)
    desc = models.CharField(max_length=100, null=False)

    def delete(self, *args, **kwargs):
        # create a new Activity
        activity = Activity()
        activity.title = self.name + " deleted!"

        # if using ForeignKey field
        activity.project = self
        # if using IntegerField
        activity.project = self.id

        super(Project, self).delete(*args, **kwargs)

    def __str__(self):
        return self.name

Note: Set on_delete=DO_NOTHING if you're using a ForeignKey field so the instance won't be deleted. (It's not a good idea since it will cause integrity issues.)

answered Jul 17, 2020 at 10:04

Navid Zarepak

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3 Comments

Sharath Nayak Over a year ago

Thank u for the answer but what would Foreign key field of the "Activity" table should contain?

Navid Zarepak Over a year ago

a Foreign key field receives an instance of a Project. I also noted that you probably should not use a foreign key field.

Sharath Nayak Over a year ago

Yes. I have tested it out. It works fine. I shouldn't have used foreign key

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