Django's built-in "last_login" field on user not being updated

[rebeccacremona]

Since this deployment in 2020, which included our upgrade from Django 1 to Django 2, the "last_login" field, part of Django's AbstractBaseUser model, updated via signal... has not been updated or populated for any user.

I have not yet diagnosed why. Our pre-save signal for Links is working, so it is not the case that all signals are broken.

A few months after that deployment, we added django-axes, so if we want, we can backfill the appropriate value of this field for any user who has logged in since 2020-10-15 16:40.

[rebeccacremona]

Okay, this happened during the upgrade to Django 2.1, because of the change introduced to Django by this PR.

It only registers the update_last_login signal if the user model's last_login is an instance of django.db.models.query_utils.DeferredAttribute.

But, we are using Django Model Utils' FieldTracker to track changes made to our user objects' fields.... so, for us,

last_login_field = getattr(get_user_model(), 'last_login', None)

is a model_utils.tracker.DescriptorWrapper, not a django.db.models.query_utils.DeferredAttribute.

Possible fixes:

• register the signal ourselves
• exclude last_login from the fields being tracked
• swap in LIL's custom field-tracking code, which Jack wrote because "those packages can be error-prone in hard-to-diagnose ways", if it works
• maybe other ideas too

I'll see what seems to be cleanest.

[rebeccacremona]

See https://github.com/jazzband/django-model-utils/issues/423 and https://github.com/jazzband/django-model-utils/issues/497

[rebeccacremona]

The tracker was added to LinkUser in https://github.com/harvard-lil/perma/commit/0b90830ae86544389852384f412ef2a85afa82bd, which added it to every model in the app at the same time. I don't see that we are actually consulting the tracker on LinkUser anywhere and all our tests pass with it removed.

So, I think we can just get rid of it!

[rebeccacremona]

Start: 62075 users with no last_login.
Updated last_login for 81324 users.
End: 51772 users with no last_login.