#29908: Blog access in b.entry_set.all()[i].blog should not hit the database
-------------------------------------+-------------------------------------
Reporter: Carsten Fuchs | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.11
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Carsten Fuchs):
Hi Tim,
it seems that this is triggered when the ForeignKey's `to_field` parameter
is used.
The test fails with the following model `EntryB`:
{{{
#!python
from django.db import models
class Blog(models.Model):
key = models.CharField(max_length=10, unique=True)
name = models.CharField(max_length=100)
def __str__(self):
return self.name
class EntryA(models.Model):
blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
headline = models.CharField(max_length=255)
class EntryB(models.Model):
blog = models.ForeignKey(Blog, on_delete=models.CASCADE,
to_field='key') # this triggers the problem
headline = models.CharField(max_length=255)
}}}
Test cases:
{{{
#!python
from django.test import TestCase
from TestApp.models import Blog, EntryA, EntryB
class TestBlogs(TestCase):
def test_A(self):
blog = Blog.objects.create(name='Beatles Blog')
EntryA.objects.create(blog=blog)
EntryA.objects.create(blog=blog)
with self.assertNumQueries(1): # succeeds
for e in blog.entrya_set.all():
self.assertEqual(e.blog.name, 'Beatles Blog')
def test_B(self):
blog = Blog.objects.create(name='Beatles Blog')
EntryB.objects.create(blog=blog)
EntryB.objects.create(blog=blog)
with self.assertNumQueries(1): # This fails!
for e in blog.entryb_set.all():
self.assertEqual(e.blog.name, 'Beatles Blog')
}}}
Test results:
{{{
$ ./manage.py test
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
.F
======================================================================
FAIL: test_B (TestApp.tests.TestBlogs)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/carsten/xxxTestProject/TestApp/tests.py", line 23, in test_B
self.assertEqual(e.blog.name, 'Beatles Blog')
File "/home/carsten/.virtualenvs/Zeiterfassung/lib/python3.6/site-
packages/django/test/testcases.py", line 88, in __exit__
query['sql'] for query in self.captured_queries
AssertionError: 3 != 1 : 3 queries executed, 1 expected
Captured queries were:
SELECT "TestApp_entryb"."id", "TestApp_entryb"."blog_id",
"TestApp_entryb"."headline" FROM "TestApp_entryb" WHERE
"TestApp_entryb"."blog_id" = ''
SELECT "TestApp_blog"."id", "TestApp_blog"."key", "TestApp_blog"."name"
FROM "TestApp_blog" WHERE "TestApp_blog"."key" = ''
SELECT "TestApp_blog"."id", "TestApp_blog"."key", "TestApp_blog"."name"
FROM "TestApp_blog" WHERE "TestApp_blog"."key" = ''
----------------------------------------------------------------------
Ran 2 tests in 0.003s
FAILED (failures=1)
Destroying test database for alias 'default'...
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/29908#comment:2>
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