How to raise a field bound `ValidationException` in django's models `clean` method?

```
from django.core.exceptions import ValidationError

def clean(self):
    if self.title:
        raise ValidationError({'title': 'not ok'})

```

The above does not add the error to the `title` field (when using a form), but to the non field errors (`__all__`).

I know how to do it inside a form (`self._errors['title'] = self.error_class([msg])`), but `self._errors` does not exist inside the models `clean` method.