Integer SITE_ID check incorrect when a different primary key type is used

[Tim Graham]

To catch a programmer mistake where the SITE_ID setting is defined with an incorrect type (e.g. SITE_ID = "1"), #31802 added a system check that only allows SITE_ID to be an int or None. This forces third-party databases with non-integer primary keys like MongoDB (which uses ObjectId) to silence this check.

It would be more appropriate if this check performed the validation based on Site._meta.pk rather than with hardcoded types.

I thought of a few options:

1. Use Site._meta.pk.to_python(settings.SITE_ID) to validate the SITE_ID, relying on it to raise ValidationError for unexpected types. This won't work because AutoField.to_python() accepts strings that coerce to integer which are values the original fix was intended to reject.

2. Use Site._meta.pk.to_python(settings.SITE_ID) but also check if settings.SITE_ID != Site._meta.pk.to_python(settings.SITE_ID). This would catch invalid values that to_python() coerces to the correct type.

3. Add Field.expected_type which could be int, ObjectId, or whatever. The implementation of the check could continue to use isinstance(). The downside is that adding a new attribute to the Field API would be non-trivial.

I've implemented option #2.

[Tim Graham]

​PR