Django unique_together with nullable ForeignKey

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Django 2.2 added a new constraints API which makes addressing this case much easier within the database.

You will need two constraints:

  1. The existing tuple constraint; and
  2. The remaining keys minus the nullable key, with a condition

If you have multiple nullable fields, I guess you will need to handle the permutations.

Here’s an example with a thruple of fields that must be all unique, where only one NULL is permitted:

from django.db import models
from django.db.models import Q
from django.db.models.constraints import UniqueConstraint

class Badger(models.Model):
    required = models.ForeignKey(Required, ...)
    optional = models.ForeignKey(Optional, null=True, ...)
    key = models.CharField(db_index=True, ...)

    class Meta:
        constraints = [
            UniqueConstraint(fields=['required', 'optional', 'key'],
                             name="unique_with_optional"),
            UniqueConstraint(fields=['required', 'key'],
                             condition=Q(optional=None),
                             name="unique_without_optional"),
        ]

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