#36372: refresh_from_db with explicit fields can't clear a relation unless it's
prefetched
-------------------------------------+-------------------------------------
Reporter: Roman Donchenko | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | 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
-------------------------------------+-------------------------------------
Consider the following project:
`testapp/__init__.py`: empty
`testapp/models.py`:
{{{
from django.db import models
class Container(models.Model):
pass
class Item(models.Model):
container = models.ForeignKey(Container, on_delete=models.CASCADE)
}}}
`testapp/migrations/__init__.py`: empty
`testapp/migrations/0001_initial.py`:
{{{
# Generated by Django 5.2 on 2025-05-06 17:03
import django.db.models.deletion
from django.db import migrations, models
class Migration(migrations.Migration):
initial = True
dependencies = [
]
operations = [
migrations.CreateModel(
name='Container',
fields=[
('id', models.AutoField(auto_created=True,
primary_key=True, serialize=False, verbose_name='ID')),
],
),
migrations.CreateModel(
name='Item',
fields=[
('id', models.AutoField(auto_created=True,
primary_key=True, serialize=False, verbose_name='ID')),
('container',
models.ForeignKey(on_delete=django.db.models.deletion.CASCADE,
to='testapp.container')),
],
),
]
}}}
`test.py`:
{{{
import django
from django.conf import settings
from django.core import management
settings.configure(
INSTALLED_APPS=["testapp"],
DATABASES={
'default': {
'ENGINE': 'django.db.backends.sqlite3',
'NAME': ':memory:',
},
},
)
django.setup()
management.call_command("migrate")
from testapp.models import Container
Container.objects.create()
c = Container.objects.first()
# c = Container.objects.prefetch_related("item_set").first()
c.refresh_from_db(fields=["item_set"])
}}}
Running `test.py` results in the following output:
{{{
Operations to perform:
Apply all migrations: testapp
Running migrations:
Applying testapp.0001_initial... OK
Traceback (most recent call last):
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/options.py", line 683, in get_field
return self.fields_map[field_name]
~~~~~~~~~~~~~~~^^^^^^^^^^^^
KeyError: 'item_set'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "[...]/test.py", line 24, in <module>
c.refresh_from_db(fields=["item_set"])
File "[...]/venv/lib/python3.12/site-packages/django/db/models/base.py",
line 737, in refresh_from_db
db_instance = db_instance_qs.get()
^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/query.py", line 629, in get
num = len(clone)
^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/query.py", line 366, in __len__
self._fetch_all()
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/query.py", line 1935, in _fetch_all
self._result_cache = list(self._iterable_class(self))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/query.py", line 91, in __iter__
results = compiler.execute_sql(
^^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/compiler.py", line 1609, in execute_sql
sql, params = self.as_sql()
^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/compiler.py", line 765, in as_sql
extra_select, order_by, group_by = self.pre_sql_setup(
^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/compiler.py", line 85, in pre_sql_setup
self.setup_query(with_col_aliases=with_col_aliases)
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/compiler.py", line 74, in setup_query
self.select, self.klass_info, self.annotation_col_map =
self.get_select(
^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/compiler.py", line 252, in get_select
select_mask = self.query.get_select_mask()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/query.py", line 885, in get_select_mask
return self._get_only_select_mask(opts, mask)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/sql/query.py", line 854, in
_get_only_select_mask
field = opts.get_field(field_name)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "[...]/venv/lib/python3.12/site-
packages/django/db/models/options.py", line 685, in get_field
raise FieldDoesNotExist(
django.core.exceptions.FieldDoesNotExist: Container has no field named
'item_set'
}}}
OTOH, if you uncomment the second-to-last line in `test.py`, there is no
exception.
I would expect the `refresh_from_db` call to simply do nothing if the
relation is not prefetched. This behavior is making it impossible to use
this functionality unless you know for sure whether the relation is
prefetched.
--
Ticket URL: <https://code.djangoproject.com/ticket/36372>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion visit
https://groups.google.com/d/msgid/django-updates/01070196aa2d81bb-77e9401b-0450-4d82-8f7c-6244d8ec88b0-000000%40eu-central-1.amazonses.com.
