#27311: Support unpickled models (e.g. read from cache) in migrations
-------------------------------------+-------------------------------------
Reporter: Odero | Owner: nobody
Type: New feature | Status: closed
Component: Migrations | Version: master
Severity: Normal | Resolution: wontfix
Keywords: migrations, | Triage Stage:
foreignkey, cache | Someday/Maybe
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Markus Holtermann):
* status: new => closed
* type: Bug => New feature
* version: 1.10 => master
* keywords: migrations, foreignkey => migrations, foreignkey, cache
* resolution: => wontfix
* stage: Unreviewed => Someday/Maybe
Old description:
> Trying to create the migration below which is failing. It seems like the
> same issue discussed and resolved in #24282.
>
> {{{
> def generate_invoices(apps, schema_editor):
> A = apps.get_model('app1', 'A')
> B = apps.get_model('app1', 'B')
> User = apps.get_model('accounts', 'User')
>
> a = A.objects.last()
>
> b = B(
> client=a.user,
> status=5
> )
> b.save()
> }}}
>
> Also tried
>
> {{{
> b = B(
> client=User.objects.get(pk=a.user.pk),
> status=5
> )
> }}}
>
> In both cases I get the error:
> {{{
> Cannot assign "<User: Some User>": "B.client" must be a "User" instance.
> }}}
New description:
Trying to create the migration below which is failing. It seems like the
same issue discussed and resolved in #24282.
{{{
def generate_invoices(apps, schema_editor):
A = apps.get_model('app1', 'A')
B = apps.get_model('app1', 'B')
User = apps.get_model('accounts', 'User')
a = A.objects.last()
b = B(
client=a.user,
status=5
)
b.save()
}}}
Also tried
{{{
b = B(
client=User.objects.get(pk=a.user.pk),
status=5
)
}}}
In both cases I get the error:
{{{
Cannot assign "<User: Some User>": "B.client" must be a "User" instance.
}}}
Investigating the issue shows that it's due to the models being read from
cache.
--
Comment:
This is not really feasible as the unpickling of models from cache uses
the global app registry
(https://github.com/django/django/blob/d5c0eb7d686ea89d6dbf787abc5bf18302c4e428/django/db/models/base.py#L1770).
We would need to look up the models from the app registry at this state of
the migration process. I have no idea how we could achieve that w/o
passing `apps` around all the time.
I also think it's not a good idea to cache objects during the migration
process. You can easily end up with objects from previous model states
that are completely invalid at a later stage because too many things
changed in between that wasn't taken care of for cached model instances.
I'm closing this as won't fix for the time being. If this is possible at
some point we could re-evaluate the feature, but for now I doubt it's
going to work out.
--
Ticket URL: <https://code.djangoproject.com/ticket/27311#comment:8>
Django <https://code.djangoproject.com/>
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