[Django] #33809: ValueError in migrations using apps.get_model for a ForeignKey
Django
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Reporter: Jesse | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 4.0
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 |
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Django 4.0.5, Python 3.9.5
I believe this is a regression of
https://code.djangoproject.com/ticket/24282
Snippet of the stack trace:
{{{
File "/home/.../venv/lib/python3.9/site-
packages/django/db/models/manager.py", line 85, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/.../venv/lib/python3.9/site-
packages/django/db/models/query.py", line 512, in create
obj = self.model(**kwargs)
File "/home/.../venv/lib/python3.9/site-
packages/django/db/models/base.py", line 541, in __init__
_setattr(self, field.name, rel_obj)
File "/home/.../venv/lib/python3.9/site-
packages/django/db/models/fields/related_descriptors.py", line 235, in
__set__
raise ValueError(
ValueError: Cannot assign "<Foo: Foo object (...)>": "Bar.foo" must be a
"Foo" instance.
}}}
This error is coming from a `RunPython()` function. It's using a model
from `apps.get_model()`, querying a model instance, and then using that
instance in a ForeignKey field. I assume the error is because the type
from `apps.get_model()` isn't the same as the actual model class.
Essentially this is what it's doing:
{{{
def custom_migration_func(apps, schema_editor):
Foo = apps.get_model("foo", "Foo")
Bar = apps.get_model("bar", "Bar")
for foo in Foo.objects.all():
Bar.objects.create(foo=foo) # breaks here
}}}
I was able to workaround this by setting the ID directly:
{{{
Bar.objects.create(foo_id=foo.pk)
}}}
If adding support for this isn't feasible, the documentation should at
least be updated to address this. I wasn't able to find any information
regarding this in the docs.
--
Ticket URL: <https://code.djangoproject.com/ticket/33809>
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