Can I change the value of a primary key with PonyORM?

Question

I need to change the original values of primary keys in a entity, but I am unable to do it. For example:

#!/usr/bin/env python3
# vim: set fileencoding=utf-8

from pony import orm

db = orm.Database("sqlite", ":memory:", create_db=True)


class Car(db.Entity):
    number = orm.PrimaryKey(str, 12)
    owner = orm.Required("Owner")


class Owner(db.Entity):
    name = orm.Required(str, 75)
    cars = orm.Set("Car")


db.generate_mapping(create_tables=True)


with orm.db_session:
   luis = Owner(name="Luis")
   Car(number="DF-574-AF", owner=luis)

with orm.db_session:
   car = Car["DF-574-AF"]
   # I try to change the primary key
   car.set(number="EE-12345-AA")

But I get a TypeError (Cannot change value of primary key attribute number).

Answer 1

The primary key should ideally be immutable. You can add an auto-incremental id to your Car class as a primary key, then make your number unique and you will be able to change it easily while still having the same constraints.

eg

class Car(db.Entity):
    id = PrimaryKey(int, auto=True)
    number = orm.Required(str, 12, unique=True)
    owner = orm.Required("Owner")