Django m2m_changed signal is never called

Question

I can't understand why my m2m_changed signal is not triggered.

Here is the code:

models.py

class Badge(TimeStampable, Expirable, Deactivable,
            SafeDeleteModel):
    _safedelete_policy = HARD_DELETE

    owner = models.ForeignKey(settings.AUTH_USER_MODEL,
                              blank=True, null=True,
                              on_delete=models.PROTECT)
    restaurants = models.ManyToManyField(Restaurant)
    identifier = models.CharField(max_length=2048)

    objects = SafeDeleteManager.from_queryset(BadgeQuerySet)()

signals.py

from django.db.models.signals import m2m_changed
from django.dispatch import receiver

from .models import Badge

@receiver(m2m_changed, sender=Badge.restaurants.through)
def my_callback(sender, **kwargs):
    print("M2M has been changed!")

apps.py (this post advised to change this file)

from django.apps import AppConfig

class BadgesConfig(AppConfig):
    name = 'badges'

    def ready(self):
        import badges.signals

When going to a shell:

• m2m_changed.receivers returns an empty list (it does not work, and the signal can never be received)

I found similar post but did not found the answer in it.

Why m2m_changed signal does not work?

EDIT

When opening a shell and importing badges.signals, it works. It means the problem is in apps.py:

In [1]: from django.db.models.signals import m2m_changed

In [2]: m2m_changed.receivers
Out[2]: []

In [3]: import badges.signals

In [4]: m2m_changed.receivers
Out[4]:
[((4551224720, 4520068792),
  <weakref at 0x10f4da5e8; to 'function' at 0x10f462d90 (check_uniqueness)>)]

Answer 1

I found my mistake in this section of Django the documentation. From the beggining, my apps configurations were never called!

Actually, to register an app correctly, I have two methods:

method 1

INSTALLED_APPS = ['badges', ...]

and declaring in __init__.py: default_app_config = 'badges.apps.BadgesConfig'

method 2

INSTALLED_APPS = ['badges.apps.BadgesConfig']

my mistake

I was using the INSTALLED_APPS = ['badges', ...] without declaring anything in __init__.py

I think maybe Django could display a warning when it notices that apps.py is in a folder app but never used.