Custom field with default unique

[petrprikryl]

Example:

class UniqueField(models.CharField):
    def __init__(self, *args, **kwargs):
        kwargs.setdefault("unique", True)
        super().__init__(*args, **kwargs)


@pghistory.track()
class Foo(models.Model):
    slug = UniqueField()

Which leads to following migration:

        migrations.CreateModel(
            name='Foo',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('slug', foo_app.models.UniqueField(unique=True)),
            ],
        ),
        migrations.CreateModel(
            name='FooEvent',
            fields=[
                ('pgh_id', models.AutoField(primary_key=True, serialize=False)),
                ('pgh_created_at', models.DateTimeField(auto_now_add=True)),
                ('pgh_label', models.TextField(help_text='The event label.')),
                ('id', models.IntegerField()),
                ('slug', foo_app.models.UniqueField(unique=True)),
            ],
            options={
                'abstract': False,
            },
        ),

I think that FooEvent shouldn't have slug field with unique constraint as it prevents multiple tracking events for same Foo record.

Next, it is working correctly for plain CharField with explicit unique=True (constraint is not present in the event model). The problem seems to be custom fields with implicit defaults which deviates from Django's defaults.

Thanks for any opinion to this topic.

[wesleykendall]

Yea, seems like pghistory does not work well with custom fields like this.

The workaround for now is to declare a custom event model https://django-pghistory.readthedocs.io/en/3.5.1/event_models/#custom-event-models and explicitly declare the field

Will leave this open as a known bug. I imagine the solution may be straightforward if someone wants to try to address it. I wont be able to get to this anytime soon

[Elkasitu]

I believe this is not a Bug in django-pghistory but rather a shortcoming of how Django generates migrations:

It will call the field's deconstruct method which hardcodes the unique default to be False for all field types, if it is the case it will not include it in the kwargs to the field in the migration.

django-pghistory forces the unique value of its event models fields to be unique=False, but as mentioned earlier this means that in the migration the AddField/AlterField operation will call the Field without the unique kwarg, meaning it will use the Field's default (True).

This should fix it (or at least it does in my case):

class UniqueField(models.CharField):
    def __init__(self, *args, **kwargs):
        kwargs.setdefault("unique", True)
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()

        if getattr(self, "_unique") is not True:
            # if the value is different than the default, then we want to add it to the migration's kwargs
            kwargs["unique"] = False

        return name, path, args, kwargs

[Elkasitu]

Without the deconstruct override, other scenarios like explicitly setting the unique attribute on the field will not be correctly recorded in the migration:

class Foo(models.Model):
    slug = UniqueField(unique=False)

Will result in:

        migrations.AddField(
            model_name='Foo',
            name='slug,
            field=UniqueField(unique=True),
        )