Overwriting Datetime is not working in Postgresql in Django - django

Question

So what I need to do is to change few rows in a model DateTime field to 40 days in the past, with Django using a PostgreSQL database. Where I choose all products with an even ID and change the date_uploaded value.
This is what I am currently doing...
from django.core.management.base import BaseCommand
from store.models import Product
import datetime
class Command(BaseCommand):
def handle(self, *args, **options):
all_products = Product.objects.all()
for product in all_products:
if product.pk % 2 == 0:
product.date_uploaded = product.date_uploaded - datetime.timedelta(40,2,0,0)
product.save()
print(product.date_uploaded)
And for some reason when I try to save the product it works with no errors but the DateTime value is not changed. Is there anything wrong with what I am doing?
this is my models.py
class Product(models.Model):
image1 = models.ImageField(upload_to="product_images", default="https://eblossomsl.s3-us-west-2.amazonaws.com/logo.png")
image2 = models.ImageField(upload_to="product_images", blank=True)
image3 = models.ImageField(upload_to="product_images", blank=True)
image4 = models.ImageField(upload_to="product_images", blank=True)
name = models.CharField(max_length=100, unique=True)
category = models.CharField(choices=CATEGORY, max_length=20, default="HRT", db_index=True)
price = models.PositiveIntegerField()
search_query = models.TextField(blank=True, null=True)
date_uploaded = models.DateTimeField(auto_now=timezone.now())
quantity_bought = models.PositiveIntegerField(default=0)
Any help would be greatly appreciated since I am not sure what I am doing wrong.

Answer 1

The problem is in auto_now argument passed into DateTimeField, this argument is responsible for changing the value of the field to the current datetime each time .save() is called on the object, i.e. every time you are running your script it sets it to current datetime and your changes are ignored. What you really need is auto_now_add which sets the value only once at the object creation.
date_uploaded = models.DateTimeField(auto_now_add=timezone.now())
Run makemigrations, apply them and run your script again.