allows_primary_key_0 is misnamed

[Claude Paroz]

In django/db/backends/__init__.py:

    # Can an object have a primary key of 0? MySQL says No.
    allows_primary_key_0 = True

This is not totally right. MySQL refuses 0 only if the primary key is an autoincrement key. See also comments in #17653. At the very least, the comment should be updated. And ideally, allows_primary_key_0 should be renamed as (for example) allows_auto_pk_0.

[Anssi Kääriäinen]

This is a really minor issue, but also an easy one to fix.

If somebody with older MySQL versions could test this it would be valuable information. I have tested this on 5.1 where id INTEGER PRIMARY KEY accepts zero as a value. Based on that marking as accepted.

I think changing the name is the correct thing to do.

[Vajrasky Kok]

PR: ​https://github.com/django/django/pull/1984

Later, in a separate ticket I will create a unit test for exercising the MySQL capability of using non-autoincrement primary key with value 0.

[Tim Graham]

I guess the only concern here is whether or not setting this attribute on your own database backend is considered private API subject to change?

[Vajrasky Kok]

This is the separate ticket for adding unit test exercising the capability of having zero non-autoincrement primary key, #21517.

[Vajrasky Kok]

@timo: or we can add another attribute (allows_auto_pk_0) and keep this one (allows_primary_key_0).

[Tim Graham <timograham@…>]

In d3cf6cfacfb828faad4f4f97c904e259304649b3:

Fixed #17713 -- Renamed BaseDatabaseFeatures.allows_primary_key_0 to allows_auto_pk_0.

MySQL does allow primary key with value 0. It only forbids autoincrement
primary key with value 0.

Thanks Claude Paroz for the report.