How to add Check Constraints for Django Model fields?

user_128028 0
Viewed 25951

While subclassing db.models.Model, sometimes it's essential to add extra checks/constraints.

For example, I have an Event model with start_date and end_date: I want to add validation into the fields or the model so that end_date > start_date.

At least I know this can be done outside the models.Model inside the ModelForm validation. But how to attach to the fields and the models.Model?

check-constraints django-forms django-models django
6 Answers

I would not put constraints like these in the save method, it's too late. Raising an exception there, doesn't help the user who entered the data in the wrong way, because it will end up as a 500 and the user won't get the form with errors back etc.

You should really check for this in the Forms/ModelForms clean method and raise a ValidationError, so form.is_valid() returns false and you can send the errors in the form back to the user for correction.

Also note that since version 1.2, Django has had Model Validation.

It would look something like this:

class Foo(models.Model):
    #  ... model stuff...
    def clean(self):
        if self.start_date > self.end_date:
            raise ValidationError('Start date is after end date')
user_1144620

As of Django 2.2, database level constraints are supported:

from django.db import models
from django.db.models import CheckConstraint, Q, F

class Event(models.Model):
    start_date = models.DatetimeField() 
    end_date = models.DatetimeField()

    class Meta:
        constraints = [
            CheckConstraint(
                check = Q(end_date__gt=F('start_date')), 
                name = 'check_start_date',
            ),
        ]
user_107462240

Do it inside your save method of your model:

def save(self, *args, **kwargs):
    if(self.end_date > self.start_date):
        super(Foo, self).save(*args, **kwargs)
    else:
        raise Exception, "end_date should be greater than start_date"
user_265920
user_41261140

Summarizing the answers from before, here is a complete solution I used for a project:

from django.db import models
from django.db.models import CheckConstraint, Q, F
from django.utils.translation import gettext_lazy as _

class Event(models.Model):
    start_date = models.DatetimeField() 
    end_date = models.DatetimeField()

    class Meta:
        constraints = [
            # Ensures constraint on DB level, raises IntegrityError (500 on debug=False)
            CheckConstraint(
                check=Q(end_date__gt=F('start_date')), name='check_start_date',
            ),
        ]

    def clean(self):
        # Ensures constraint on model level, raises ValidationError
        if self.start_date > self.end_date:
            # raise error for field
            raise ValidationError({'end_date': _('End date cannot be smaller then start date.')})

Too bad there is no django.core.validators that can handle this :(

user_34610030
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