如何拥有一个跟踪数据库更改的外部脚本？

Question

I am using Django and have a skeleton website with a single button that increments an IntegerField in my database. I want to have an external script which continually reads this IntegerField and reports when a change has been made.

models.py

class Counter(models.Model):
    count = models.PositiveIntegerField(default=0)

views.py

def index(request):
    ctr = Counter.objects.get(id=1)
    pre = ctr.count
    print(f"pre: {pre}")

    if request.method == 'POST':
        if request.POST.get("increment"):
            Counter.objects.filter(id=1).update(count=F('count') + 1)
            post = ctr.count
            print(f"pre: {pre}")


return render(request, 'increment.html')

I thought I could simply achieve this inside the view function but I've had difficulty detecting a change because the pre & post values above are the same when I print them even though post is clearly created after the field is incremented.

I have a watcher.py file inside my app directory where I am trying to program a script that would run in the background and report changes to the database. Here's what I have so far:

from django.conf import settings

settings.configure(DEBUG=True)

import django
django.setup()

import models
import time
from ahelper import settings
from threading import Thread


def watcher():
    ctr = models.Counter.objects.get(id=1)
    pre = ctr.count
    print(f"{pre}")
    print("start loop")
    while (1):
        temp = time.time()
        post = ctr.count
        if pre != post:
            print("Change detected")
            response_time = (time.time() - temp) * 1000
            print(f"{response_time}ms")

t = Thread(target=watcher, daemon=True)
t.start()

When I try to run this, I get the error: Model class models.Counter doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS . I don't understand why this comes up and I don't know how to resolve it; I don't think making Counter it's own app is suitable.

I am very lost at this point so any help is appreciated. Am I going about this the right way? Is there an easier way to do this?

Answer 1

You must run this script in a django app. This could be done in different manner, but a way more simple one would be to use the before/post save signal of Django, so you know when this field is modified: https://docs.djangoproject.com/en/4.1/ref/signals/#django.db.models.signals.post_save

If you still want to use a dedicated runner, you can create a thread/process that run aside Django. Still you have one issue in your code,

post = ctr.count

This will not reload the object state from the database. You need to reload the object manually to ensure it is uptodate. Other way you would not detect any changes:

ctr.refresh_from_db()
post = ctr.count