Select related executes queries - django

Question

I can't make select_related work with the following configuration:
Models:
class Text(models.Model):
author = models.ForeignKey('authors.Author',
on_delete=models.SET_NULL,
blank=True,
null=True)
Author model:
class Author(models.Model):
name = models.CharField(max_length=200)
def get_absolute_url(self):
kwargs = {'pk': self.pk}
return reverse('author-detail', kwargs=kwargs)
View
In a view, I use the select_related function to avoid hitting the db when querying for text's authors e.g.:mytext.author:
class TextsViewTest(TestCase):
def text_view(request,
pk,
template_name='texts/detail.html'):
source_text = Text.objects.select_related('author').get(pk=pk)
return render(request, template_name,
{
'source': source_text,
})
Test
According to select_related it shouldn't hit the database when accessing the Text.author relationship, but when testing it with:
def test_layout_content_header__uses_prefetched_relationships(self):
author = Author.objects.create(name="foobar")
source_text = Text.objects.create(author=author)
context = {'source': source_text}
with self.assertNumQueries(0):
from django.template.loader import render_to_string
rendered = render_to_string("text/_content_header.html", context)
template
text/content_header.html:
{% if source.author %} by <em>{{source.author.name}}</em>{% endif %}
Output
./manage test texts.test_views shows a hit:
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
F
======================================================================
FAIL: test_layout_content_header__uses_prefetched_relationships (author.tests.test_views.TextsViewTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/.../text/tests/test_views.py", line 1035, in test_layout_content_header__uses_prefetched_relationships
source_text.author
File "/.../lib/python3.6/site-packages/django/test/testcases.py", line 80, in __exit__
'%d. %s' % (i, query['sql']) for i, query in enumerate(self.captured_queries, start=1)
AssertionError: 1 != 0 : 1 queries executed, 0 expected
Captured queries were:
1. SELECT "authors_author"."id", "authors_author"."name", FROM "authors_author" WHERE "authors_author"."id" = 1
----------------------------------------------------------------------
Ran 1 test in 0.489s
FAILED (failures=1)
Destroying test database for alias 'default'...
Any ideas?

Answer 1

It looks like you are not using your view's code inside the test. Try either to copy same query into your test, e.g.:
context = {'source': Text.objects.select_related('author').get(pk=source_text.pk)}
with self.assertNumQueries(0):
from django.template.loader import render_to_string
rendered = render_to_string("text/_content_header.html", context)
Or reuse the view code (it seems to be declared in the Test Case, right?)
with self.assertNumQueries(1):
self.text_view(MagicMock(), source_text.pk)
Although you might need to specify bit more advanced request mock, e.g. using the RequestFactory