[FIXED] How to know when chained django lifecycle hooks/signals are finished running?

Issue

I have an Animal model and a Herd model that looks like this:

from django_lifecycle.hooks import AFTER_CREATE

class Animal(models.Model):
    parent = OneToOneField("self", related_name="child")
    age = IntegerField()

class Herd(models.Model):
    animal = OneToOneField(Animal)
    total_age = IntegerField()
    
    @hook(AFTER_CREATE)
    def create_animal_child(self):
        if self.animal.child.exists():
            Herd.objects.create(parent=self, animal=self.animal.child)

    def save_total_age(self):
        total_age = self.animal.age
        position = self.animal
        while position.child.exists():
            total_age += position.child.age
            position = position.child

The idea being that you create your Animals. Then you create your Herd for the topmost animal you want. The Herd's AFTER_CREATEwill then create a "chain" of all the Herd objects for the Animal's children, the children's children, and so on.

E.g.

a1 = Animal.objects.create(age=10)
a2 = Animal.objects.create(parent=a1, age=7)
a3 = Animal.objects.create(parent=a2, age=3)

h1 = Herd.objects.create(animal=a1)
# Two Herd objects with an "animal" field pointing to a2 and a3 respectively are automatically created

h1.save_total_age() # h1.total_age = 20 # 10 + 7 + 3

Which is all fine.

My problem is for my front-end, how can I tell when all the relevant Animal AFTER_CREATE hooks have finished running so I know to call my calculate_total_age method?

So say I have the following REST API endpoints:

example.com/api/animals/<pk>
example.com/api/herds/<pk>
example.com/api/herds/<pk>/save_total_age

If I do a POST to example.com/api/herds/<pk> to create a Herd, I call save_total_age() as soon as that "parent" Herd is created - and not necessarily after all the "children" Herds have been created via the AFTER_CREATE hook.

axios.post(
    "example.com/api/herds/",
    {animal_id: 1}
).then((response) => {
    axios.post(
        `example.com/api/herds/${response.data.pk}/save_total_age`,
        {},
    )
});

How can I set my backend signals and/or frontend async/await so that save_total_age() only runs after my AFTER_CREATE chain is complete?

Edit: My best solution right now is to add some condition to save_word_count() that makes sure the Herd chain has the same # of objects as the Animal chain. If so, run the method. If not, return a bad request and retry the POST.

Solution

Signals (which are called by those hooks) run synchronously, not asynchronously. So when you call create() for your first heard, it will always run the AFTER_CREATE hook and create the 2nd Heard object, before it returns and let you run the save_total_age() method.

In other words this should work just fine as it is.

Answered By - Erik Kalkoken