django-xvalidate

django-xvalidate allows you to declare cross-field validators within a Django model.

As an example, consider a Django model named Event.

from django.db import models

class Event(models.Model):
    title = models.CharField(max_length=255)
    start_date = models.DateField()
    end_date = models.DateField()

django-xvalidate allows you to declare that the start date precedes the end date as follows:

from django.db import models
from xvalidate import XValidatedModel, XLe

class Event(XValidatedModel, models.Model):
    title = models.CharField(max_length=255)
    start_date = models.DateField()
    end_date = models.DateField()

    class XVMeta(XValidatedModel.XVMeta):
        spec = [
            XLe('start_date', 'end_date').message(
                'The start date should precede the end date')
        ]

XValidatedModel ensures that this specification is maintained invoking Event.clean() and raises ValidationError as appropriate.

Why use django-xvalidate?

django-xvalidate allows you to specify how to validate your model instances in a more declarative manner than writing imperative code within your clean() methods. Without django-xvalidate you would have to implement the above example as

def clean(self):
    super(Event, self).clean()
    if (self.start_date is not None) and (self.end_date is not None):
        if (self.end_date < self.start_date):
            raise ValidationError('The start date should precede the end date')

With a more declarative format we have the option at some point in the future to automate the creation of test data that passes (or fails) validation.

django-xvalidate comes some operator overloading that brings syntactic sugar to your declarations making them very easy to read. For instance, you could specify:

((XF('end_date') - 'start_date') > datetime.timedelta(days=4)).message(
    'Event should last at least 5 days'
)

django-xvalidate also allows the use of Django's double-underscore (__) syntax to dereference related objects, enabling succinct definitions such as the following

(XF('registration_date') <= 'event__end_date').message(
    'Must register before the event ends')