wagtail.contrib.forms.models.AbstractForm subclasses without a template attribute cannot be instantiated in migrations

[jams2]

Issue Summary

wagtail.contrib.forms.models.AbstractForm subclasses without a template attribute cannot be instantiated in migrations.

gitpod /workspace/wagtail-gitpod (main) $ ./manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, home, sessions, taggit, wagtailadmin, wagtailcore, wagtaildocs, wagtailembeds, wagtailforms, wagtailimages, wagtailredirects, wagtailsearch, wagtailusers
Running migrations:
  Applying home.0007_migrate_form_page...Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/base.py", line 106, in wrapper
    res = handle_func(*args, **kwargs)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/core/management/commands/migrate.py", line 356, in handle
    post_migrate_state = executor.migrate(
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/migrations/executor.py", line 135, in migrate
    state = self._migrate_all_forwards(
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/migrations/executor.py", line 167, in _migrate_all_forwards
    state = self.apply_migration(
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/migrations/executor.py", line 252, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/migrations/migration.py", line 132, in apply
    operation.database_forwards(
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/migrations/operations/special.py", line 193, in database_forwards
    self.code(from_state.apps, schema_editor)
  File "/workspace/wagtail-gitpod/home/migrations/0007_migrate_form_page.py", line 8, in migrate_form_page
    for page in FormPage.objects.all():
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/models/query.py", line 398, in __iter__
    self._fetch_all()
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/models/query.py", line 1881, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/models/query.py", line 122, in __iter__
    obj = model_cls.from_db(
  File "/workspace/.pip-modules/lib/python3.8/site-packages/django/db/models/base.py", line 582, in from_db
    new = cls(*values)
  File "/workspace/.pip-modules/lib/python3.8/site-packages/wagtail/contrib/forms/models.py", line 178, in __init__
    name, ext = os.path.splitext(self.template)
AttributeError: 'FormPage' object has no attribute 'template'

There are scenarios where users might not explicitly declare template or landing_page_template attributes:

• they are happy with the default naming scheme
• they are using Wagtail in a headless architecture

Steps to Reproduce

1. Start a new project with wagtail start myproject
2. Edit models.py as follows

from django.db import models

from wagtail.models import Page
from wagtail.contrib.forms.models import AbstractForm


class HomePage(Page):
    pass

class FormPage(AbstractForm):
    pass

3. ./manage.py makemigrations
4. ./manage.py migrate
5. Sign in to admin and create an instance of FormPage.
6. ./manage.py makemigrations --empty -n migrate_form_page home
7. Edit the new, empty migration:

# Generated by Django 4.2.11 on 2024-04-30 16:33

from django.db import migrations


def migrate_form_page(apps, _):
    FormPage = apps.get_model("home", "FormPage")
    for page in FormPage.objects.all():
        continue


class Migration(migrations.Migration):
    dependencies = [
        ("home", "0006_formpage"),
    ]

    operations = [
        migrations.RunPython(migrate_form_page, migrations.RunPython.noop),
    ]

8. ./manage.py migrate

This will cause the error.

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.8.12
• Django version: 4.2.11
• Wagtail version: 6.0.2
• Browser version: N/A

Working on this

I am happy to submit a patch for this issue