signals.py

from django.db import transaction
from django.db.models.signals import post_save
from django.dispatch import receiver

from .models import ActionLogEntry
from .models import EventLogEntry

@receiver(post_save, sender=ActionLogEntry, dispatch_uid="event_logger")
def log_action_event(sender, instance, **kwargs):
    try:
        with transaction.atomic():
            EventLogEntry.objects.get(my_model_id=instance.my_model_id)
    except EventLogEntry.DoesNotExist:
            from app.models import MyModel
            m = MyModel.objects.get(pk=instance.my_model_id)
            EventLogEntry.objects.create(my_model=m)

tests.py

from django.test import TestCase

from .models import ActionLogEntry
from .models import EventLogEntry


class TestSignalReceivers(TestCase):

    def test_action_triggers_event_log(self):
        # Create an instance of my_model
        m = MyModel.objects.create()
        # Verify that no event logs exist for this instance
        self.assertEqual(EventLogEntry.objects.filter(my_model=m).count(), 0)
        # Log an event
        EventLogEntry.objects.create(my_model=m, event="foo")
        # Log an action
        ActionLogEntry.objects.create(my_model=m, event="bar")
        # Verify that an EventLogEntry was also created for the first action
        self.assertEqual(EventLogEntry.objects.filter(my_model=m).count(), 2,
            "Initial action did not generate an EventLogEntry entry")