ModelForms always set self.instance even when none passed in

[Steven Mapes]

I came across this bug this morning when running my testsuite against Django 3.2, 4.0 and 4.1 (4.1.0, 4.1.1 and 4.1.2). The issue is with ModelForms when they are used without passing in an existing saved instance. I thought this was related to the regression that was then fixed in 4.1.2 and mentioned in both 33984 and 33952 but the issue is still occurring even though both of those are closed.

To test you can use the below models, form and unit test

# models.py
from django.db import models


class Tag(models.Model):
    tag = models.SlugField(max_length=64, unique=True)


class Thing(models.Model):
    tag = models.ForeignKey(Tag, on_delete=models.CASCADE, related_name="things")
    active = models.BooleanField(default=True)

# forms.py
from django import forms
from example.models import Tag


class TagForm(forms.ModelForm):
    class Meta:
        model = Tag
        fields = ["tag"]

    def __init__(self, *args, **kwargs):
        super(TagForm, self).__init__(*args, **kwargs)

        if self.instance and self.instance.things:
            inactive_things = self.instance.things.filter(active=False)
            # Do something with it

from unittest.case import TestCase
from example.forms import TagForm


class TagFormCase(TestCase):
    def test_required(self):
        """Test required fields"""
        form = TagForm({})
        self.assertFalse(form.is_valid())
        self.assertEqual(
            {
                "tag": ["This field is required."],
            },
            form.errors,
        )

If you run the test on Django versions <4.1 including Django 2.*, Django 3.* and 4.0.* then it'll pass but on 4.1.* it'll fail with ValueError: 'Tag' instance needs to have a primary key value before this relationship can be used.

In order to pass the if statement needs to be changed to if self.instance.pk and self.instance.things as self.instance is no longer None if no instance is passed in.

If your model uses a custom primary key such as a uuid4 then it'll work as expected as the uuid4 is already called

[Mariusz Felisiak]

Thanks for the report, however this behavior was intentionally changed in 7ba6ebe9149ae38257d70100e8bfbfd0da189862 and it's documented in release notes. Previously an empty queryset was implicitly returned for unsaved instances now you need to protect your code and check self.instance.pk.

There is no change in the ModelForm behavior, self.instance was always set.

[Steven Mapes]

Replying to Mariusz Felisiak:

Thanks for the report, however this behavior was intentionally changed in 7ba6ebe9149ae38257d70100e8bfbfd0da189862 and it's documented in release notes. Previously an empty queryset was implicitly returned for unsaved instances now you need to protect your code and check self.instance.pk.

There is no change in the ModelForm behavior, self.instance was always set.

Ah yes I missed that entry in the release notes when I was checking. Sorry about that