Watching for ForeignKey value changes seems to not trigger the hook

Using the below example in version 0.6.0, I am expecting to print a message any time someone changes a user’s first name that is saved in SomeModel. From what I can tell from the documentation, I am setting everything up correctly. Am I misunderstanding how this works?

Assuming a model set up like this:

from django.conf import settings
from django.db import models
from django_lifecycle import LifecycleModel, hook

class SomeModel(LifecycleModel):
    user = models.ForeignKey(
        on_delete=models.CASCADE,
        to=settings.AUTH_USER_MODEL
    )
    # More fields here...

    @hook('after_update', when='user.first_name', has_changed=True)
    def user_first_name_changed(self):
        print(
            f"User's first_name has changed from "
            f"{self.initial_value('user.first_name')} to {user.first_name}!"
        )

When we then perform the following code, nothing prints:

from django.contrib.auth import get_user_model

# Create a test user (Jane Doe)
get_user_model().objects.create_user(
    username='test', 
    password=None, 
    first_name='Jane', 
    last_name='Doe'
)

# Create an instance
SomeModel.objects.create(user=user)

# Retrieve a new instance of the user
user = get_user_model().objects.get(username='test')

# Change the name (John Doe)
user.first_name = 'John'
user.save()

# Nothing prints from the hook

In the tests, I see that it is calling user_account._clear_watched_fk_model_cache() explicitly after changing the Organization.name. However, from looking at the code, I do not see this call anywhere except for the overridden UserAccount.save() method. Thus, saving the Organization has no way to notify the UserAccount that a change has been made, and therefore, the hook cannot possibly be fired. The only reason that I can see that the test is passing is because of the explicit call to user_account._clear_watched_fk_model_cache().

    def test_has_changed_is_true_if_fk_related_model_field_has_changed(self):
        org = Organization.objects.create(name="Dunder Mifflin")
        UserAccount.objects.create(**self.stub_data, organization=org)
        user_account = UserAccount.objects.get()

        org.name = "Dwight's Paper Empire"
        org.save()
        user_account._clear_watched_fk_model_cache()
        self.assertTrue(user_account.has_changed("organization.name"))

[rsinger86]

I think I know what’s happening. You want a model’s hook to fire, upon making updates to its FK-related model. This isn’t supported. The hook will only fire when you make the change to the model itself: the SomeModel instance in your example.

from django.contrib.auth import get_user_model

# Create a test user (Jane Doe)
get_user_model().objects.create_user(
    username='test', 
    password=None, 
    first_name='Jane', 
    last_name='Doe'
)

# Create an instance
some_instance = SomeModel.objects.create(user=user)

# Retrieve a new instance of the user
user = get_user_model().objects.get(username='test')

# Change the name (John Doe)
user.first_name = 'John'
user.save()

# The hook will fire and it will print the message
some_instance.save()

Maybe the docs should be updated to clarify? I’ll think a little more about supporting this behavior down the line. Though, to be honest, I’m not quite sure if the “FK changes” feature should be included in the first place because of lack of support for handling it efficiently, ORM-wise.

[rsinger86]

Apologies for the radio silence - I appreciate the thought here. I just don’t think I want to expand the scope of this project to support a feature of this complexity, given that it seems to be a pretty rare use case. Furthermore, handling chains of changes like this with application code in-memory seems a little shaky… I think it would be better solved with database triggers/functions.