`default_access_fn` always returns True for forked apps

Issue Summary

I’ve managed to reproduce this for any dashboard app and will use the Pages dashboard app as the example in this ticket.

I’ve narrowed the issue down to apps.get_containing_app_config returning Shop when it should return PagesDashboardConfig. https://github.com/django-oscar/django-oscar/blob/fe983bf7140457d06094d843da0b94ebaea4d6c8/src/oscar/apps/dashboard/nav.py#L98

I’ve tried changing the check on line 99 to

if app_config_instance is None or not isinstance(app_config_instance, OscarDashboardConfig):

But then, get_app_config_class will return the base OscarDashboardConfig class which is also not what we want.

Steps to Reproduce

1. Fork the Pages dashboard app ./manage.py oscar_fork_app pages_dashboard . oscar_forks.dashboard.pages
2. Log into the dashboard with a user with partner.dashboard_access
3. Notice the Content submenu is visible while the whole Pages app has this default permission https://github.com/django-oscar/django-oscar/blob/fe983bf7140457d06094d843da0b94ebaea4d6c8/src/oscar/apps/dashboard/pages/apps.py#L13

Technical details

• Python version: 3.7.4
• Django version: 2.2.3
• Oscar version: 2.0

[rik]

Here’s my PRs plan:

• default_access_fn https://github.com/django-oscar/django-oscar/pull/3087

• Simplify forking command to only override the name attribute https://github.com/django-oscar/django-oscar/pull/3084

• Document the need to fork top-level apps before nested apps Alternatively, we could change the command when forking a nested app to either:

  • warn if the top-level app hasn’t been forked yet
  • fork the top-level app if needed

  But to keep things manageable, I’ll probably just document for now.

[solarissmoke]

We have a bunch of forked apps in the _site project that is used for tests. One approach is to fork one of the dashboard apps into there, and then have integration tests that check that the forked app’s config is being used to determine permissions.

This approach is not particularly pretty because the forking applies for the entire test suite, but we haven’t found a better alternative.

This would ideally be in addition to unit tests for _dashboard_urls_to_config() for which get_app_configs() can be mocked.