如何更新 Django 中的相关 model 字段

Question

I have two models (purchase and stock), I Want when i purchase an item the quantity field in stock to be updated. (eg. i have 2 Mangoes in stock and i purchase 3 Mongoes, i want the updated stock to be 5 Mangoes)

My Models

class Stock(models.Model):
    product = models.ForeignKey(Products, null=True, blank=True, on_delete = models.SET_NULL)
    stock_date = models.DateField(null=True, blank=True)
    quantity = models.DecimalField(max_digits = 10, decimal_places = 2, blank=True)
    buying_price = models.DecimalField(max_digits = 10, decimal_places = 2, blank=True)

class Purchase(models.Model):
    product = models.ForeignKey(Stock, null=True, blank=True, on_delete = models.SET_NULL)
    order_date = models.DateField(null=True, blank=True)
    quantity = models.DecimalField(max_digits = 10, decimal_places = 2, blank=True)
    buying_price = models.DecimalField(max_digits = 10, decimal_places = 2, blank=True)

Can anyone help please!

Answer 1

You can access Django models like any variable

 CurrentPurchase.stock.quantity = 5

or you can use filter to get the object you want:

Model.objects.filter(AnotherModel=Model_)

Answer 2

You should use Django Signals.

from django.db.models.signals import post_save

def update_stock(sender, instance, **kwargs):
    product = instance.product.id    
    stock = Stock.objects.get(product=product.id)
    stock.quantity = stock.quantity + instance.quantity

post_save.connect(update_stock, sender=Purchase)

You can add the exceptions according to your needs.