I have a class, which represents photos attached to a person.
class PersonPhoto(models.Model): id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False) photo = models.FileField(upload_to='photos/', blank=True, null=True) person = models.ForeignKey( Person, related_name="photos", on_delete=models.CASCADE ) main = models.BooleanField(default=False)
I want to make sure that for every person there could be only one main photo. In pure SQL i would use something like
create unique index on photo (person_id, main) where main = true;
You can play with it here http://sqlfiddle.com/#!15/34dfe/4
How to express such constraint in django model? I am using django 4.0.1
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Answer
You can add a UniqueConstraint to your model Meta.constraints with a condition to enable this constraint
class PersonPhoto(models.Model): id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False) photo = models.FileField(upload_to='photos/', blank=True, null=True) person = models.ForeignKey( Person, related_name="photos", on_delete=models.CASCADE ) main = models.BooleanField(default=False) class Meta: constraints = [ models.UniqueConstraint(fields=['person'], condition=models.Q(main=True), name='unique_person_main'), ]