[nonrel] Support for non-integer AutoFields

[Jonas H.]

(This patch is part of the changes made to the inofficial fork "Django-nonrel" which adds basic support for non-relational databases.)

This patch adds basic support for non-integer primary keys. It works by not restricting primary keys to int and factoring out pk validation/type conversion into the backend.

[Carl Meyer]

Accepting the ticket, as support for non-integer auto-fields is certainly desirable. I believe there's a different implementation of it in Alex Gaynor's 2010 GSoC code - I don't know which implementation is preferable.

[Russell Keith-Magee]

IIRC, Alex's GSoC code punted on this issue (or, at least, had code that wasn't suitable for trunk), because we couldn't come up with a way to introduce it without introducing backwards incompatibilities. I seem to recall there were a bunch of places in the code that assumed that AutoFields were integers; however, it's been a while, so I might be mis-remembering.

[Jonas H.]

A few contrib apps assume PKs to be integers and also it gets tricky when it comes to testing with fixtures. Should we address all these issues at once or fix them bit by bit in multiple tickets?

[Aymeric Augustin]

Two tickets (#17214, #17122) were recently reported about problems in the admin with models using non-integer primary keys. A quick search for "primary_key" turns up several problems with non-integer primary keys, but I think these should be handled separately.

I'm updating the title to reflect that this ticket is about non-integer automatic primary keys.

[Nick Pope]

This is partially addressed by ​PR for #29979, but will further depend on #470 for default values for non-integer fields as well as #24042 for replacing isinstance(..., AutoField) checks.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 21e5594:

Fixed #29979, Refs #17337 -- Extracted AutoField field logic into a mixin and refactored AutoFields.

This reduces duplication by allowing AutoField, BigAutoField and
SmallAutoField to inherit from IntegerField, BigIntegerField and
SmallIntegerField respectively. Doing so also allows for enabling the
max_length warning check and minimum/maximum value validation for auto
fields, as well as providing a mixin that can be used for other possible
future auto field types such as a theoretical UUIDAutoField.