python 37 lines · 7 steps

Enforcing one default address per user in Django

A Django model uses a partial unique constraint so each user can have only one default address of each kind.

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Django Intermediate data modeling database constraints partial index enum choices
1from django.db import models
2from django.db.models import Q
3from django.conf import settings
4 
5 
6class Address(models.Model):
7 class Kind(models.TextChoices):
8 SHIPPING = "shipping", "Shipping"
9 BILLING = "billing", "Billing"
10 
11 user = models.ForeignKey(
12 settings.AUTH_USER_MODEL,
13 on_delete=models.CASCADE,
14 related_name="addresses",
15 )
16 kind = models.CharField(max_length=16, choices=Kind.choices)
17 line1 = models.CharField(max_length=255)
18 line2 = models.CharField(max_length=255, blank=True)
19 city = models.CharField(max_length=120)
20 postal_code = models.CharField(max_length=20)
21 country = models.CharField(max_length=2)
22 is_default = models.BooleanField(default=False)
23 
24 class Meta:
25 constraints = [
26 models.UniqueConstraint(
27 fields=["user", "kind"],
28 condition=Q(is_default=True),
29 name="unique_default_address_per_user_kind",
30 ),
31 ]
32 indexes = [
33 models.Index(fields=["user", "kind"]),
34 ]
35 
36 def __str__(self):
37 return f"{self.get_kind_display()} address for {self.user_id}"
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A UniqueConstraint with a condition becomes a partial index, enforcing uniqueness only for rows that match.
  2. 2TextChoices gives you a validated enum plus a human-readable display method for free.
  3. 3Pushing business rules like 'one default per kind' into the database prevents race conditions application code can't reliably catch.

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