[Django] #33323: On custom migrations, import of model classes with app.get_model('appname', 'ModelName') ignores overwritten models.Model methods

[Django]

#33323: On custom migrations, import of model classes with app.get_model('appname',
'ModelName') ignores overwritten models.Model methods
--------------------------------------+------------------------
Reporter: ascheucher | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 3.2
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
--------------------------------------+------------------------
Given migration:

{{{
from django.db import migrations
from designs.models import Design

def fix_wrong_design_sheet_data(apps, schema_editor):
# do not import Model classes like in the line below. Overwritten
methods are not executed.
# Design = apps.get_model('designs', 'Design')

for design in Design.objects.all().iterator():
if design. uuid in ['79f53b09-7069-480e-a8d9-d88ec0aad8d0',
'e26230cb-a9c7-4ee1-9523-0f2f70cf5c98']:
design.language = 'nt'
design.save(
force_insert=False,
force_update=True,
using='default',
update_fields=['id_slug', 'language'])

class Migration(migrations.Migration):

dependencies = [
('designs',
'0027_related_design_cols_to_many_many_remove_old_col'),
]

operations = [
migrations.RunPython(fix_wrong_design_sheet_data),
]
}}}

Model class:

{{{
class Design(models_mixins.PkUUID, models_mixins.TrackingMixin):
niche = models.CharField(blank=False, null=True, max_length=50)
id = models.CharField(blank=False, null=True, max_length=4)
variant = models.CharField(blank=False, null=True, max_length=1)
language = models.CharField(max_length=2, default=None)

# this field is filled by the .save() method
id_slug = models.CharField(max_length=61, blank=False, null=True)
...

def save(self, force_insert: bool, force_update: bool, using:
Optional[str], update_fields: Optional[Iterable[str]]) -> None:
print('new save')
int_id = int(self.id)
nslid =
f'{self.niche}-{int_id:04}-{self.variant}--{self.language}'
self.id_slug = nslid
return super().save(force_insert=force_insert,
force_update=force_update, using=using, update_fields=update_fields)
}}}

When imported with `from designs.models import Design`, then the
Design.save method is called.

BUG:
Importing the Design model class with `Design = apps.get_model('designs',
'Design')` the Design.save method is not called.

--
Ticket URL: <https://code.djangoproject.com/ticket/33323>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#33323: On custom migrations, import of model classes with app.get_model('appname',
'ModelName') ignores overwritten models.Model methods

-----------------------------------+--------------------------------------
Reporter: Andreas Scheucher | Owner: nobody
Type: Bug | Status: closed
Component: Migrations | Version: 3.2
Severity: Normal | Resolution: invalid

Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0

-----------------------------------+--------------------------------------
Changes (by Tim Graham):

* status: new => closed
* resolution: => invalid

Comment:

[https://docs.djangoproject.com/en/stable/topics/migrations/#historical-
models As documented]:

> Because it’s impossible to serialize arbitrary Python code, these
historical models will not have any custom methods that you have defined.
They will, however, have the same fields, relationships, managers (limited
to those with use_in_migrations = True) and Meta options (also versioned,
so they may be different from your current ones).
>
> This means that you will NOT have custom save() methods called on
objects when you access them in migrations, and you will NOT have any
custom constructors or instance methods.

--
Ticket URL: <https://code.djangoproject.com/ticket/33323#comment:1>