Django / postgres - maintain a table of counters - django

Question

I want to maintain a running counter per user, that will be given to the user items.
User table will have a item_counter column
class Item:
name = models.CharField(max_length=100)
user = models.ForeignKey(User, on_delete=models.CASCADE)
item_num = models.IntegerField() # should be populated from the user table
The question, when the user is creating an item, what is the best way to increment the counter and to copy the new counter to the newly created item, atomically?
Technically I atomically need to:
Increment and get the counter of the user
Create a new Item with the value for item_counter
What is the django / postgres way to do that?
EDIT - example:
Users table:
| username | item_counter |
| john | 20 |
Now, the user sends POST /item. The result will be:
Items table:
| name | item_num |
| my_item | 21 |
Users table:
| username | item_counter |
| john | 21 |
Of course, there can be many users. Each user has an item_counter of its own.

Answer 1

From what I understand you need to assign a special integer to your newly created item record whose value will be fetched from a user's counter variable. The user's counter variable will increment every time the user creates a new Item record.
So as per your challenge this is your solution.
In your class Item declare a method for incrementing the value of counters
from django.db import transaction, IntegrityError #Django inbuilt atomicity transaction block
class Item(models.Model):
name = models.CharField(max_length=100)
user = models.ForeignKey(User, on_delete=models.CASCADE)
item_num = models.IntegerField()
def increment_counter(self):
temp = self #Here I have used temp so that it does not throw an out of reference error in your atomic block below
with transaction.atomic():
# This code executes inside a transaction.
temp.user.counter = temp.user.counter + 1
temp.user.save()
temp.item_num = temp.user.counter
temp.save()
In your views.py POST method once you saved/created the new Item you need to call the item.increment_counter() method
newly_created_item = Item.objects.create(....some data)
try:
newly_created_item.increment_counter()
except IntegrityError:
# Handle any exception here