class Calculation(models.Model):
  release       = models.OneToOneField("Release", on_delete=models.SET_NULL, blank=True, null=True, related_name="calculation")
class Release(models.Model):
  def save(self, *args, **kwargs):
    super().save(*args, **kwargs)
    if not self.calculation:
        Calculation.objects.create(release=self)

class BaseModelAdmin(admin.ModelAdmin):
    def save_model(self, request, obj, form, change):
        user = request.user
        instance = form.save(commit=False)
        if not change or not instance.user:
            instance.created = datetime.datetime.now()
            instance.user = user
        elif instance.user:
            instance.edited = datetime.datetime.now()
        try:
            instance.save()
            form.save_m2m()
        except Exception as err:
            self.message_user(request, str(err), level=messages.ERROR)
            return HttpResponseRedirect("..")
        return instance

    def save_formset(self, request, form, formset, change):
        instances = formset.save(commit=False)
        for obj in formset.deleted_objects:
            obj.delete()
        for instance in instances:
            instance.created = datetime.datetime.now()
            instance.user = request.user
            try:
                instance.save()
            except Exception as err:
                self.message_user(request, str(err), level=messages.ERROR)
                return HttpResponseRedirect("..")
        try:
            formset.save_m2m()
        except Exception as err:
            self.message_user(request, str(err), level=messages.ERROR)
            return HttpResponseRedirect("..")

if not getattr(self, 'calculation'):
   Calculation.objects.create(release=self)