Django pre_delete signal gets ignored

Question

Here is what is run on pre_delete of model Document. This code gets royally ignored when put in a separate file (signals.py) as suggested best practice. When put in model file, it works fine.

from django.db.models.signals import pre_delete, pre_save
from django.dispatch import receiver
from _myTools.functions import ImageProcessing
from content_image.models import Document
import os

# Image deletion when deleting entire entry
@receiver(pre_delete, sender=Document, dispatch_uid='document_delete_signal')
def entry_deletion_images_delete(sender, instance, using, **kwargs):
    for key, value in instance.imageSizes.items():
        name_of_image_field = str(getattr(instance, key))  # Converts to string, if not is object itself
        os.remove(instance.baseDir + name_of_image_field)
        setattr(instance, key, None)

So what is the problem ? Should I import something more in there ? Or should I import this file somewhere ?

score 4 · Accepted Answer · edited Jun 07 '16 at 06:40

4

The problem is that if you put it in a signals.py (as recommended) but do nothing more, then nobody will be importing that file.

You should follow this advice, in particular

In practice, signal handlers are usually defined in a signals submodule of the application they relate to. Signal receivers are connected in the ready() method of your application configuration class. If you’re using the receiver() decorator, simply import the signals submodule inside ready().

The ready() is reponsible for importing the module and thus provoking the signal "connection" and allows the signal receiver to work.

The full procedure should be:

# my_app/__init__.py

default_app_config = 'my_app.apps.ConfigForMyApp'

and the apps.py should be:

# my_app/apps.py

from django.apps import AppConfig

class ConfigForMyApp(AppConfig):
    # Optionally add `name` and `verbose_name`
    # for the app
    def ready(self): # overriding the ready method
        # This will trigger the @receiver decorator 
        # and thus connect the signals
        import my_app.signals

edited Jun 07 '16 at 06:40

Algorithmatic

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answered Jan 22 '16 at 13:57

MariusSiuram

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Thanks it works. Is it worth the extra complexity ? is there anything wrong in leaving it in the models ? – Robert Brax Jan 22 '16 at 14:17
Depends on how much you plan to grow your app. If it won't grow, then leave it in the `models`. If you plan to redistribute it, use a bunch of migration stuff, some strange connections, add unit tests, etc. then it probably will be worth doing it, because you may avoid some bugs and the code will be more clean and tidy. – MariusSiuram Jan 22 '16 at 14:25
Thanks, and if I want it to be system wide (not related to specific app), I should put them not in app directory, but in project apps.py and __init__ ? – Robert Brax Jan 22 '16 at 14:34

score 2 · Answer 2 · answered Jan 22 '16 at 14:30

Though the answer provided by MariusSiuram is the new recommended way for django >= 1.7, here is another pattern which would work irrespective of the version of django:

In your models.py file:

from django.db.models.signals import pre_delete
from myapp.signals import pre_delete_signal_dispatcher

pre_delete.connect(pre_delete_signal_dispatcher, sender=MyModel, dispatch_uid="delete_signal_dispatch")

This way, the signals are connected when models.py file is parse, and triggered on pre_delete event, and you can continue to grow your signals.py file. But yes, if the number of signal connections grow, it is easier to manage them in the app config.

this expects also to remove the `@received` decorator, doesn't it? It indeed provides more clean code, although the potential problems of signal connection on model loading may yet be a problem. However, +1 :) — MariusSiuram, Jan 22 '16 at 14:52

Django pre_delete signal gets ignored

2 Answers2