Model._perform_unique_checks cannot properly validate simultaneous adding and changing

[Roman]

Imagine there is a model with a unique field.

class Test(models.Model):
    index = models.PositiveIntegerField()
    name = models.CharField(max_length=256)

    class Meta:
        constraints = (
            models.UniqueConstraint(
                fields=('index'),
                name='%(app_label)s_%(class)s_U1'),
        )

If I have one model with values (0, "old") and then try to update existing model to (1, "old") and simultaneously add (0, "new"), checks in Model._perform_unique_checks fail, despite there will be no constraint violation at the end of transaction.

[Simon Charette]

Does the problem manifests itself if you use define your unique constraint using index = models.PositiveIntegerField(unique=True) instead of relying on Meta.constraints?

[Tim Graham]

#25139 might be related or a duplicate.

[Simon Charette]

yeah that was my thought as well, I'll close for now as it seems like just another flavor of #25139 using a new API.