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Django model constraints

Django models and forms provide various options how to validate incoming data and catch data inconsistency. One can set up such things like:

form field validations
form complex (multi-field) validations
model validations
model complex (multi-field) validations
additional manual validations in view

Those are okay, but not perfect for all possible cases. The reason is form validators can be bypassed in cases where forms and ORM models are not used - like admin (assume here you did not specify extra form for admin) or - and the most likely - direct manipulation with the database. Like dump restoration or import from another (3rd party) tool - like psql. Those cases are out of Django app control and the only way is to have some kind of "validations" directly in the database. That's when constraints come to the game.

Constraints

Django does support model constraints. That means that any constraint defined on a model is later reflected to the database via database migrations and sits there. If you attempt to violate your rules by inserting directly into the database you will get caught.

Let's assume this simple model which represents Facebook's OG tags.

class OgTag(models.Model):
    page = PageField(blank=True, null=True )
    url = models.CharField(blank=True, max_length=255)
    name = models.CharField(max_length=50)
    value = models.TextField(blank=True, null=True)
    image = FilerImageField(blank=True, null=True, on_delete=models.CASCADE)
    is_default = models.BooleanField(default=False)

    objects = OgTagManager()

    def get_image(self):
        if self.image:
            return self.image.url

Let's assume the following rules:

page and url are similar fields
page is a list of site pages and url is for arbitrary URL
both are mutually exclusive
but one of them is required
is_default is unique in a combination with value
value and image are mutually exclusive
but one of them is required

Those rules can be enforced quite easily with form clean() validation method. But you can also set up model constraints that will be in sync with your database:

class OgTag(models.Model):
    ...

    class Meta:
        constraints = [
            models.CheckConstraint(check=~Q(value="") | Q(image__isnull=False), name="image_or_value"),
            models.CheckConstraint(check=Q(value="") | Q(image__isnull=True), name="image_or_value_not_both"),
            models.CheckConstraint(check=~Q(url="") | Q(page__isnull=False), name="page_or_url"),
            models.CheckConstraint(check=Q(url="") | Q(page__isnull=True), name="page_or_url_not_both"),
            models.UniqueConstraint(fields=["name"], condition=Q(is_default=True), name="unique_defaults"),
        ]

Now you just need to run database migrations and you are all set up.

Error messages

One (big) downside that comes with constraints is that Django cannot catch, parse and map database errors to some sane error messages, so you just get regular database errors which lead to HTTP 5XX error code. To prevent this, you need to reflect those constraint rules to a form validation where you can construct user-friendly error messages. Then you are fully set up.