> I think we should restrict the setting between normal and big auto fields 
> only. Allowing UUID's would be changing the type, with the potential for 
> havoc with code incompalities throughout django. It's also not possible to 
> migrate tables over to the new type.

That’s a really good point, I got a bit swept up with the idea to realise that 
it would break a lot of things if applied generally!

> The autodetector knows if a model is new. It could be that during one version 
> Django outputs BigAutoField for fields added in CreateModel only.

We could make every automatic PK become a BigAutoField but include some logic 
in the migrations framework to ignore changes between an auto_created AutoField 
-> BigAutoField? This would ignore all existing models in your migration 
history, but all completely new models would receive BigAutoField PKs from the 
start. Users who explicitly add a `BigAutoField` to a model with a previously 
created `AutoField` would see a migration created as the `auto_created` flag is 
not set. It would also play nicely with foreign keys which also need to be 8 
bytes.

This would take care of third party apps with migrations and not require a 
setting but the downside is that model state will be slightly different from 
the database. All models would have `BigAutoField`’s whereas the database would 
only have an `AutoField`. This feels slightly iffy to me even though I cannot 
think of a case where it would have any practical effect.

> On 11 Jun 2020, at 19:22, Adam Johnson <m...@adamj.eu> wrote:
> 
> Big +1 on solving this from me.
> 
> - The setting would take any dotted path to a class, or a single class name 
> for a build in field. This would potentially solve [3], and could be useful 
> to people who want to default to other fields like UUIDs (or a custom 
> BigAutoField) for whatever reason
> 
> I think we should restrict the setting between normal and big auto fields 
> only. Allowing UUID's would be changing the type, with the potential for 
> havoc with code incompalities throughout django. It's also not possible to 
> migrate tables over to the new type.
> 
> What do you think the solution is for third-party apps? They ship their own 
> migrations and can't really be tied to project state.
> 
> As Django migrations are derived from the current model state so there’s no 
> way I can think of to express “make this auto-generated field a BigAutoField 
> only if this model is new”.
> 
> The autodetector knows if a model is new. It could be that during one version 
> Django outputs BigAutoField for fields added in CreateModel only.
> 
> On Thu, 11 Jun 2020 at 16:28, Tom Forbes <t...@tomforb.es 
> <mailto:t...@tomforb.es>> wrote:
> I’d like to re-propose switching Django to use BigAutoField’s rather than the 
> current AutoField. This has been proposed[1] before (and a MR made[2]) but it 
> was closed due to implementation issues and not much else has happened since 
> then.
> 
> As many of you are aware the max value a standard AutoField can hold is 
> 2,147,483,647 (2.1 billion) which sounds like more than you can ever need. 
> But it’s often not, and you only find out at the worst possible time - out of 
> the blue at night and during a period of rapid growth. The process for fixing 
> this issue also becomes a lot harder as your data grows - when you’ve hit the 
> limit you’re looking at a multi-hour `ALTER TABLE` on Postgres during which 
> writes and reads are blocked, or a risky operation to create a new table with 
> the correct primary key and copy old data over in batches. Basically if 
> you’ve experienced this before you wouldn’t wish it on your worst enemy.
> 
> I’m proposing that we add a `MODELS_PRIMARY_KEY` (name improvements welcome!) 
> setting that _defaults_ to `BigAutoField`, with prominent docs/release notes 
> saying that to preserve the existing behaviour this should be set to 
> `AutoField`. I think this the only realistic way we can implement this for 
> new projects in a way that ensures it will be used meaningfully and not 
> forgotten about until it’s too late.
> 
> Rails managed to do this migration somewhat painlessly due the big 
> differences between Rails and Django models. As Django migrations are derived 
> from the current model state so there’s no way I can think of to express 
> “make this auto-generated field a BigAutoField only if this model is new”. 
> The way I see it is that a global setting is very easy to toggle and there is 
> little chance of missing the large numbers of migrations that would be 
> generated during the Django update. Smaller applications could apply the 
> migrations with little issue and larger applications would be able to opt-out 
> (as well as be reminded that this is a problem they could face!).
> 
> Some specifics:
> - The setting would take any dotted path to a class, or a single class name 
> for a build in field. This would potentially solve [3], and could be useful 
> to people who want to default to other fields like UUIDs (or a custom 
> BigAutoField) for whatever reason
> - The setting would also be used for GenericForeignKeys, which right now are 
> backed by a PositiveIntegerField and so suffer from the same AutoField 
> limitations
> 
> Any thoughts on this?
> 
> Tom
> 
> 1. https://groups.google.com/d/msg/django-developers/imBJwRrtJkk/P4g0Y87lAgAJ 
> <https://groups.google.com/d/msg/django-developers/imBJwRrtJkk/P4g0Y87lAgAJ>
> 
> 2. https://github.com/django/django/pull/8924/ 
> <https://github.com/django/django/pull/8924/>
> 
> 3. https://code.djangoproject.com/ticket/56 
> <https://groups.google.com/d/msg/django-developers/VFXZpHnuEJc/bbefjX9yCQAJ>
> 
> 
> 
> 
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> -- 
> Adam
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