How can I enforce a minimum age constraint and manage related models in Django?

Question

I am working on a Django project where I need to validate a model before saving it, based on values in its related models. I came up with this issue while extracting an app from an project using an old Django version (3.1) to a separate Django 5.1 project, then there error "ValueError: 'Model...' instance needs to have a primary key value before this relationship can be used" raised on all validation classes that used related model data.

For demonstration and simplification purposes, I have a Reservation model that references multiple Guest objects via a foreign key. For the reservation to be valid and be saved, all guests linked to it must be at least 18 years old.

However, none of these records (neither the reservation nor the guests) have been saved to the database yet. I need to perform this validation efficiently and cleanly, preferably in a way that keeps the validation logic separated from the models themselves.

How can I approach this validation scenario? What are the best practices for validating unsaved foreign key relationships in Django?

Here is a simplified version of my setup:

File: models.py

from django.db import models

class Reservation(models.Model):
    check_in_date = models.DateField()
    check_out_date = models.DateField()

    def __str__(self):
        return f"Reservation from {self.check_in_date} to {self.check_out_date}"

class Guest(models.Model):
    name = models.CharField(max_length=255)
    age = models.PositiveIntegerField()
    reservation = models.ForeignKey(
        Reservation,
        related_name="guests",
        on_delete=models.CASCADE
    )

    def __str__(self):
        return f"{self.name} ({self. Age} years old)"

File: validation.py

from django.core.exceptions import ValidationError

def validate_reservation_and_guests(reservation):
    """
    Validate that all guests in the reservation are at least 18 years old.
    """
    for guest in reservation.guests.all():
        if guest.age < 18:
            raise ValidationError("All guests must be at least 18 years old.")

Question:

What is the best way to structure this kind of validation in Django admin? I am open to using custom model methods, form validation, or signals, but I prefer to keep the logic in a separate file for better organization. Are there other approaches I should consider?

Any examples or advice would be greatly appreciated!

Answer 1

Multi-model validation can (technically) happen in several places:

• Model(s)
• Form/modelform
• View

Models generally validate single-instance values, whereas it might be more usual for forms to validate criteria that span several instances and/or models. Evaluating instance data directly in the view is usually an anti-pattern.

It comes down to use-case and preference. If you need different multi-level validations on the same model(s) in different scenarios, you'd probably do it in the form. If you need the same validation regardless of how/when/etc the model is accessed, it makes a lot of sense to put it on the model.

In the case you describe I would prefer doing it on the model. While you could conceivably achieve the same with forms, the complexity is probably higher due to juggling instances of different models (and their respective forms) inline.

Here's a crude reference implementation:

class Guest(models.Model):
    ...

    def clean(self):
        from .validation import validate_reservation_and_guests
        validate_reservation_and_guests(self.reservation)

ModelForm calls Model.clean(), and (as other answers also have pointed out) the admin panel uses ModelForm, so this will be sufficient to have the validation triggered.