[Answered ]-Having a unqiue counter per User for a different model in Django

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By the "unique counter per User" you mean the code series per User. So I would create a new model ReceiptCodeSeries

from django.db import transaction, models


class ReceiptCodeSeries(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    series_count = models.IntegerField(default=0)

    # Optional fields
    prefix = models.CharField(default="2021")
    suffix_length = models.IntegerField(default=4)

    @classmethod
    def get_next_code(cls, user):
        with transaction.atomic():
            series = cls.objects.select_for_update().get(user=user)
            series.series_count += 1
            series.save()
            return series.prefix + str(series.ticket_series_count).zfill(series.suffix_length)

This approach locks the code series for each get_next_code call so parallel requests will be fine.

Example usage:

>>> code_series = ReceiptCodeSeries.objects.get(user=some_user)
>>> print(code_series.series_count)
0
>>> ReceiptCodeSeries.get_next_code(some_user)
"2021-0001"
>>> code_series.refresh_from_db()
>>> print(code_series.series_count)
1

If you want to learn more about this approach, please read a great article from Haki Benita – How to Manage Concurrency in Django Models

👤Kryštof Řeháček

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