Should default model fields be set by the Form or the Model?

Question

Which option is best, 1 or 2?

1.

class TopicForm(forms.Form):

    name = forms.CharField(required=True)
    body = RichTextFormField(required=True)

    def save(self, request):
        t = models.Topic(user=request.user,
                         site=get_current_site(request),
                         name=self.cleaned_data['name'],
                         body=self.cleaned_data['body'])
        t.slug = slugify(self.name)
        t.body_html = seo.nofollow(seo.noindex(self.body))
        t.ip = utils.get_client_ip(request)
        t.save()

or 2.

class Topic(models.Model):
    ...   

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        self.body_html = seo.nofollow(seo.noindex(self.body))
        self.ip = utils.get_client_ip(request)
        super(Topic, self).save(*args, **kwargs)

Answer 1

The difference is that the first version is only applied when modifying objects through the form, while the second is applied whenever the model is saved (though that is still a subset of all the ways in which database rows can be modified in Django). Even if you currently only create objects through forms, I think it's still a useful distinction to keep in mind.

It looks to me like a mixture of the two makes sense in your case. A slug is something that you will always want to set based on name - that is, it's inherent to the model itself. On the other hand, the idea of a client_ip seems inexorably tied to the notion of creating an object with a form via a web request.

Of course, you are in a better position to know about the specifics of this model, but that is the general way I would approach the question.