Django: How to automatically update a model instance everytime another model instance is updated-CodePudding

In online shop projects, I have a model called Order (and this model stores order instances) and a model called Points. The model Points stores bonus points which are collected by the users when they make orders. But the order may be cancelled, so I would like to be able to monitor when an order is being cancelled (model Order instance's status being changed to "cancelled") in order to take the points away. How can I do that?

CodePudding user response：

Use Django signals: https://docs.djangoproject.com/en/4.0/ref/signals/#django.db.models.signals.post_save

You can detect that Order instance was saved and which field exactly was updated and then do some logic when thsi happens.

Example:

from django.db.models.signals import post_save
from django.dispatch import receiver
from myapp.models import Order


@receiver(pre_save, sender=Order)
def post_save_order_handler(sender, instance, created, **kwargs):
    if not created:
    for item in iter(kwargs.get('update_fields')):
        if item == 'status' and instance.status == "cancelled":
           # do something here

Or you can similarily use pre_save signal and check if status changed from some valid status to cancelled before saving.

CodePudding user response：

can you share your models and functions? you can easily implements two more queries to maintain points for user using placing order and cancelling order Functions. for example

def place_order(request):
       Order.objects.create()
       Point.objects.create(user_id=user_id,points="10")

vice versa for Cancelling order update status of order and update the points to 0 or minus from total.