Django Simple Model Versioning

forms.py

class NewsStoryForm(forms.ModelForm):

    # ...form fields

    def is_valid(self):
        valid = super(NewsStoryForm, self).is_valid()
        story = self.instance
        form_version = self.cleaned_data.get('version', None)
        latest_version = Story.objects.get(pk=story.pk).version

        if not valid:
            return valid

        if form_version != latest_version:
            print(form_version, latest_version)
            self._errors['version_error'] = 'Wrong version and stuff'
            return False
        return True

models.py

class Page(models.Model):

    # ... model fields

    version = models.PositiveIntegerField(default=1)

    def save(self, *args, **kwargs):
        if self.id:
            self.version += 1
        super(Page, self).save(*args, **kwargs)