Cannot resolve a custom through model passed as app_label.ModelName for m2m relations

[antoineauger]

PS: this is a corner case of #1093. I made some experiments in #1149 before creating this issue because I don't have the required knowledge to fix it.

Describe the bug

django-simple-history cannot resolve a custom through model passed as app_label.ModelName for m2m relations.

To Reproduce

When trying to pass the Membership model as a tests.Membership string, this produces an error when testing inside the project repository with python3 runtests.py:

class Membership(models.Model):
    group = models.ForeignKey("Group", on_delete=models.CASCADE)
    person = models.ForeignKey("Person", on_delete=models.CASCADE)
    inviter = models.ForeignKey(
        "Person",
        on_delete=models.CASCADE,
        related_name="membership_invites",
    )
    invite_reason = models.CharField(max_length=64)


class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(
        "Person",
        through="tests.Membership",
        through_fields=("group", "person"),
    )

    history = HistoricalRecords(m2m_fields=[members])

Test output:

python3 runtests.py                                                             
Traceback (most recent call last):
  File "/home/antoine/_Github/django-simple-history/runtests.py", line 193, in <module>
    main()
  File "/home/antoine/_Github/django-simple-history/runtests.py", line 180, in main
    django.setup()
  File "/home/antoine/.local/lib/python3.10/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/antoine/.local/lib/python3.10/site-packages/django/apps/registry.py", line 116, in populate
    app_config.import_models()
  File "/home/antoine/.local/lib/python3.10/site-packages/django/apps/config.py", line 269, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/antoine/_Github/django-simple-history/simple_history/tests/models.py", line 159, in <module>
    class Group(models.Model):
  File "/home/antoine/.local/lib/python3.10/site-packages/django/db/models/base.py", line 365, in __new__
    new_class._prepare()
  File "/home/antoine/.local/lib/python3.10/site-packages/django/db/models/base.py", line 428, in _prepare
    class_prepared.send(sender=cls)
  File "/home/antoine/.local/lib/python3.10/site-packages/django/dispatch/dispatcher.py", line 176, in send
    return [
  File "/home/antoine/.local/lib/python3.10/site-packages/django/dispatch/dispatcher.py", line 177, in <listcomp>
    (receiver, receiver(signal=self, sender=sender, **named))
  File "/home/antoine/_Github/django-simple-history/simple_history/models.py", line 223, in finalize
    m2m_model = self.create_history_m2m_model(
  File "/home/antoine/_Github/django-simple-history/simple_history/models.py", line 258, in create_history_m2m_model
    fields = self.copy_fields(through_model)
  File "/home/antoine/_Github/django-simple-history/simple_history/models.py", line 330, in copy_fields
    for field in self.fields_included(model):
  File "/home/antoine/_Github/django-simple-history/simple_history/models.py", line 313, in fields_included
    for field in model._meta.fields:
AttributeError: 'str' object has no attribute '_meta'

Expected behavior

It correctly works when passing the Membership model as a Python class:

class Membership(models.Model):
    group = models.ForeignKey("Group", on_delete=models.CASCADE)
    person = models.ForeignKey("Person", on_delete=models.CASCADE)
    inviter = models.ForeignKey(
        "Person",
        on_delete=models.CASCADE,
        related_name="membership_invites",
    )
    invite_reason = models.CharField(max_length=64)


class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(
        "Person",
        through=Membership,
        through_fields=("group", "person"),
    )

    history = HistoricalRecords(m2m_fields=[members])

Environment (please complete the following information):

• OS: Ubuntu 22.04.3 LTS
• Django Simple History Version: 3.4.0
• Django Version: 4.2.5

[antoineauger]

/cc @ddabble

[antoineauger]

This is still a problem despite the fixes brought by release 3.5.0.

[ddabble]

Yeah, this wasn't addressed in 3.5.0, but it's on my personal TODO list - provided that no one fixes it before I get to it 🙂

[skyward-luke]

any updates on this?

[trumpet2012]

I have a proposed fix for this here: #1390. Feedback and any additional testing are welcome.

[Arti3DPlayer]

+1, not working with through models defined as string

[whiffsalot]

I have a proposed fix for this here: #1390. Feedback and any additional testing are welcome.

Would love to see this get merged! This was blocking us from having history working reliably on some of our models.

[nagababa123]

m2m_fields is working with through if your model class is defined like this i.e. non string format:

slides = models.ManyToManyField( "MetaSlide", related_name="+", through=MetaSlideCourse, )