Django 1.8: How can I ensure that of Two Fields in a Model, At Least One or Only One must meet a condition?

Question

For context, here is a menu system.

class Menu(models.Model):
    ...


class Link(models.Model):
    ...


class MenuItem(models.Model):
    menu = models.ForeignKey(Menu)
    submenu = models.ForeignKey(Menu, related_name='submenu', blank=True, null=True)
    link = models.ForeignKey(Link, blank=True, null=True)
    position = models.IntegerField()

I have two results I'm looking to achieve:

At least one of Submenu and Link must not be Null (submenu titles can have a link)
Only one of Submenu and Link must be null (submenu titles cannot have a link)

Any advanced validation is new to me, so a code example would be very helpful.

In this example, data will only be added via Django Admin

I haven't tried this, so I won't make it an answer, but perhaps you could use multi-table inheritance? If you make both `Menu` and `Link` subclass `MenuItem` then `MenuItem` will have two one to one relationships only one of which is non-null. — Duncan, Aug 03 '15 at 13:08
Thanks, that sounds like a potentially neater way to get the second result, but wouldn't cover the first. — StringsOnFire, Aug 03 '15 at 13:38

score 11 · Accepted Answer · edited Sep 30 '19 at 19:42

The documentation around model validation is poor. There are numerous (closed) issues referring to it, but it's still unclear.

This solution works, without making changes to any Forms:

from django.core.exceptions import ValidationError

class MenuItem(models.Model):
    ...

    def clean(self):
        super(MenuItem, self).clean()
        if self.submenu is None and self.link is None:
            raise ValidationError('Validation error text')

clean() has some default validation functionality, so the clean belonging to Model needs be called first.

The above ensures that at least one of the two fields are used, and raises the exception if not. I have only tested this in the Admin interface.

I don't know if this is the correct way to do this, and would love to know more if someone has a better understanding of model validation in Django. Coming from another languages and frameworks, this does feel like the natural way to write custom validation.

how to do same check if one of these fields is a Many-To-Many field? — mh-firouzjah, Jun 12 '20 at 08:49

Django 1.8: How can I ensure that of Two Fields in a Model, At Least One or Only One must meet a condition?

1 Answers1