How to create a unique constraint for a associated model, transaction management

Using Django Using the ORM
1

I have two models:

class DesiredTenant(models.Model):
    _DEFAULT_MINIMUM_PASSWORD_LENGTH: int = 8

    id = models.CharField("System-ID", primary_key=True, max_length=63, unique=True,
                          validators=[ALLOWED_CHARACTERS_REGEX_VALIDATOR])
....

and

class Domain(models.Model):
    system = models.ForeignKey(DesiredTenant, on_delete=models.CASCADE)
    domain = models.CharField(max_length=200, unique=True)
    primary_domain = models.BooleanField("Primary Domain", default=False)
....

I am using django-admin to interactively create a new tenant in my user-interface.
The user defines the “id” in the user interface and adds a new domain as a (django.contrib.admin.) StackedInline.

I would like to add a constraint that only allows one domain to be the primary_domain for a specific tenant (ideally selected by a radio button in the StackedInline).

Is that possible? How can i implement that?

I suppose i have two problems:

  • how can i achieve the radio button selection in the StackedInlines
  • how can i define a suitable constraint?
  • how can i build a transactional context in the database (postgresql) for all Domain objects associated to the tenant to prevent constraint violations? (by default django executes the save() method independently for every domain)
2

This is one of those situations where I think you’ll really be better off writing your own form and view to process it. It’s likely to be a lot easier than trying to make this work within the admin.

From the docs for The Django admin site:

If you need to provide a more process-centric interface that abstracts away the implementation details of database tables and fields, then it’s probably time to write your own views.