Fix changing PK via admin

[Csirmaz Bendegúz]

I have noticed two bugs when changing the primary key field via admin.

Given:

# models.py
class Dummy(models.Model):
    id = models.IntegerField(primary_key=True)

# admin.py
admin.site.register(Dummy)

1. Create Dummy (1), Dummy (2).
2. Change Dummy (2)'s Id field -> 1
3. Dummy (1) is overwritten with Dummy (2), Dummy(2) stays unchanged.
4. Change Dummy (1)'s Id field -> 3
5. A new Dummy (3) is created, Dummy (1) stays unchanged.

I believe both step 3 and step 5 are bugs.

[Sarah Boyce]

Great catch thank you!

I believe step 5 should never happen, since step 3 should fail with a unique check error.

Agreed 👍 if you were to add another field to the model then you slightly override Dummy (1) with whatever is in Dummy (2) - which is not nice.

[Csirmaz Bendegúz]

So actually there are 2 issues:

1. The unique check is skipped when PK is changed.
2. The object is duplicated when PK is changed.

I'm looking into this.

[Csirmaz Bendegúz]

I have submitted a basic fix for raising a ValidationError on PK conflict.

The other issue I would like to address is the fact that Django creates a duplicate if there's no PK conflict.

Should I create another ticket for this?

[Csirmaz Bendegúz]

I submitted another PR for the above, please review both! Thanks!

[Csirmaz Bendegúz]

I edited the ticket to reflect my findings.