Correct Way to Validate Django Model Objects?

Question

I'm still trying to understand the correct way to validate a Django model object using a custom validator at the model level. I know that validation is usually done within a form or model form. However, I want to ensure the integrity of my data at the model level if I'm interacting with it via the ORM in the Python shell. Here's my current approach:

from django.db import models
from django.core import validators
from django.core exceptions import ValidationError


def validate_gender(value):
    """ Custom validator """
    if not value in ('m', 'f', 'M', 'F'):
        raise ValidationError(u'%s is not a valid value for gender.' % value)


class Person(models.Model):
    name = models.CharField(max_length=128)
    age = models.IntegerField()
    gender = models.CharField(maxlength=1, validators=[validate_gender])

    def save(self, *args, **kwargs):
        """ Override Person's save """
        self.full_clean(exclude=None)
        super(Person, self).save(*args, **kwargs)

Here are my questions:

1. Should I create a custom validation function, designate it as a validator, and then override the Person's save() function as I've done above? (By the way, I know I could validate my gender choices using the 'choices' field option but I created 'validate_gender' for the purpose of illustration).

2. If I really want to ensure the integrity of my data, should I not only write Django unit tests for testing at the model layer but also equivalent database-level unit tests using Python/Psycopg? I've noticed that Django unit tests, which raise ValidationErrors, only test the model's understanding of the database schema using a copy of the database. Even if I were to use South for migrations, any database-level constraints are limited to what Django can understand and translate into a Postgres constraint. If I need a custom constraint that Django can't replicate, I could potentially enter data into my database that violates that constraint if I'm interacting with the database directly via the psql terminal.

Thanks!

Answer 1

I had a similar misunderstanding of the ORM when I first started with Django.

1. No, don't put self.full_clean() inside of save. Either

A) use a ModelForm (which will cause all the same validation to occur - note: ModelForm.is_valid() won't call Model.full_clean explicitly, but will perform the exact same checks as Model.full_clean). Example:

class PersonForm(forms.ModelForm):
    class Meta:
        model = Person

def add_person(request):
    if request.method == 'POST':
        form = PersonForm(request.POST, request.FILES)
        if form.is_valid():  # Performs your validation, including ``validate_gender``
            person = form.save()
            return redirect('some-other-view')
    else:
        form = PersonForm()
        # ... return response with ``form`` in the context for rendering in a template

Also note, forms aren't for use only in views that render them in templates - they're great for any sort of use, including an API, etc. After running form.is_valid() and getting errors, you'll have form.errors which is a dictionary containing all the errors in the form, including a key called '__all__' which will contain non-field errors.

B) Simply use model_instance.full_clean() in your view (or other logical application layer), instead of using a form, but forms are a nice abstraction for this.

2. I don't really have a solution to, but I've never run into such a problem, even in large projects (the current project I work with my company on has 146 tables) and I don't suspect it'll be a concern in your case either.

Answer 2

As of django 4.1, self.full_clean by default also runs self.validate_constraints (docs).

After makemigrations, there will be a migrations.AddConstraint statement included in the migration.

In your case:

from django.db import models

class Person(models.Model):
    class Gender(models.TextChoices):
        M = "M", "Male"
        F = "F", "Female"

    name = models.CharField(max_length=128)
    age = models.IntegerField()
    gender = models.CharField(maxlength=1, choices=Gender.choices)

    class Meta:
        constraints = [
            models.CheckConstraint(
                check=models.Q(gender__in=Gender.values), name="valid_gender"
            ),
            models.CheckConstraint(
                check=models.Q(age__gte=18), name="adults_only"
            ),
        ]

After running migrations, if you now try to self.save(), will raise an IntegrityError!