Limit the number of records in a Model that can have a value per user

Question

I have a Model with a boolean field named is_active. I want to limit the number of models a user can have where the value of that boolean field is True to 1.

class MyModel(models.Model):
    user = models.ForeignKey(User)
    is_active = models.BooleanField(default=False)
    #...more fields ...

Initially I was just going to add a unique_togeather = ("user", "is_active") entry to Meta -- but of course that will also limit the number of False entries a User can have -- which I need to be unlimited.

Ideally, I'd like to solve this at the database level to prevent race conditions.

The models are being created by Celery tasks importing data using MyModel.objects.get_or_create() -- so it is very possible that a race condition could occur due to the level of concurrency from Celery workers.

Answer 1

You should create custom clean method on your model.

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

class MyModel(models.Model):
    user = models.ForeignKey(User)
    is_active = models.BooleanField(default=False)
    #...more fields ...

    def clean(self):
        if not self.pk and MyModel.objects.filter(user=self.user, is_active=True).exists():
            raise ValidationError('How about no?')