from django.db import models
from django.db.models import F


class Square(models.Model):
    side = models.IntegerField()
    area = models.GeneratedField(
        expression=F("side") * F("side"),
        output_field=models.BigIntegerField(),
        db_persist=True,
    )

https://docs.djangoproject.com/en/5.1/releases/5.0/#database-generated-model-field