Getting django signals to work between applications

Question

I have a Django app called receipt_printer which should be printing an order when it is received using a signal. There are two apps here called: Orders and receipt_printer.

However, whenever an order is processed and fully charged I can't get the signals to work. The example below just prints out something to the console for simplicity. If I do everything inside the models.py in the Orders app it works fine.

receipt_printer/apps.py

from django.apps import AppConfig

class ReceiptPrinterConfig(AppConfig):
    name = 'receipt_printer'

    def ready(self):
        from . import signals

receipt_printer/signals.py

from django.db import models
from saleor.order.models import Order
from django.db.models.signals import post_save

def order_fully_paid_signal(sender, instance, created, **kwargs):
    if instance.get_payment_status() == "fully-charged":
        print("This signal is working for fully paid order")
    else:
        print("This signal won't working for fully paid order")
post_save.connect(order_fully_paid_signal, sender=Order) 

receipt_printer/init.py

default_app_config = 'receipt_printer.apps.ReceiptPrinterConfig'

UPDATE, also tried the below - didn't work:

@receiver(post_save, sender=Order)
def order_fully_paid_signal(sender, instance, created, **kwargs):
    if instance.get_payment_status() == "fully-charged":
        print("This signal is working for fully paid order")
    else:
        print("This signal won't working for fully paid order")

Answer 1

I needed to add the following line to the bottom of my models.py for the Orders app:

from saleor.saleor_package.receipt_printer.signals import order_fully_paid_signal