[Django] #34288: Using queryset manager methods as default value breaks makemigrations

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Jan 24, 2023, 12:29:52 AM1/24/23

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#34288: Using queryset manager methods as default value breaks makemigrations
----------------------------------------------+----------------------------
Reporter: Mikhail Podgurskiy | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 4.1
Severity: Normal | Keywords: migrations
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
----------------------------------------------+----------------------------
When I'm using a manager method as default value for a model field,
migrations can't be generated

{{{
class Company(models.Model):
name = models.CharField(max_length=250)

class Client(models.Model):
name = models.CharField(max_length=250)
company = models.ForeignKey(
Company,
on_delete=models.DO_NOTHING,
default=Company.objects.last
)

}}}

{{{
django/db/migrations/serializer.py", line 169, in serialize
raise ValueError(
ValueError: Could not find function last in django.db.models.manager.

}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/34288>
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Django

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Jan 24, 2023, 3:23:52 AM1/24/23

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#34288: Using queryset manager methods as default value breaks makemigrations

-------------------------------------+-------------------------------------

Reporter: Mikhail Podgurskiy | Owner: nobody

Type: Bug | Status: closed
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution: duplicate

-------------------------------------+-------------------------------------
Changes (by Mariusz Felisiak):

* status: new => closed
* resolution: => duplicate
* component: Migrations => Database layer (models, ORM)

Comment:

This was fixed in 46efd03d2674084051f7ce04006e95520649a1ac. However, as
[https://docs.djangoproject.com/en/stable/ref/models/fields/#default
documented]:
> ''"The default can’t be a mutable object (**model instance**, list, set,
etc.), ..."''
> ''"For fields like ForeignKey that map to model instances, defaults
should be the value of the field they reference (pk unless to_field is
set) **instead of model instances**."''
so the default cannot be a model instance.