Django test setup fails when running single test file

Question

The immediate symptoms:

$ pytest …/apps/users/test/test_graph_permissions_command.py
============================= test session starts ==============================
platform linux -- Python 3.6.8, pytest-4.6.2, py-1.8.0, pluggy-0.12.0
Django settings: dms.settings (from ini file)
rootdir: /src/app, inifile: pytest.ini
plugins: forked-1.0.2, celery-4.3.0, django-3.5.0, cov-2.7.1, xdist-1.28.0
collected 1 item                                                               

…/apps/users/test/test_graph_permissions_command.py E

==================================== ERRORS ====================================
_ ERROR at setup of TestGraphPermissionsCommand.test_should_resolve_a_permission_hierarchy _
.venv/lib/python3.6/site-packages/pytest_django/plugin.py:519: in _django_setup_unittest
    request.getfixturevalue("django_db_setup")
.venv/lib/python3.6/site-packages/pytest_django/fixtures.py:108: in django_db_setup
    **setup_databases_args
.venv/lib/python3.6/site-packages/django/test/utils.py:174: in setup_databases
    serialize=connection.settings_dict.get('TEST', {}).get('SERIALIZE', True),
.venv/lib/python3.6/site-packages/django/db/backends/base/creation.py:80: in create_test_db
    self.connection._test_serialized_contents = self.serialize_db_to_string()
.venv/lib/python3.6/site-packages/django/db/backends/base/creation.py:123: in serialize_db_to_string
    serializers.serialize("json", get_objects(), indent=None, stream=out)
.venv/lib/python3.6/site-packages/django/core/serializers/__init__.py:128: in serialize
    s.serialize(queryset, **options)
.venv/lib/python3.6/site-packages/django/core/serializers/base.py:90: in serialize
    for count, obj in enumerate(queryset, start=1):
.venv/lib/python3.6/site-packages/django/db/backends/base/creation.py:120: in get_objects
    yield from queryset.iterator()
.venv/lib/python3.6/site-packages/django/db/models/query.py:341: in _iterator
    yield from self._iterable_class(self, chunked_fetch=use_chunked_fetch, chunk_size=chunk_size)
.venv/lib/python3.6/site-packages/polymorphic/query.py:56: in _polymorphic_iterator
    o = next(base_iter)
.venv/lib/python3.6/site-packages/django/db/models/query.py:55: in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:1087: in execute_sql
    sql, params = self.as_sql()
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:474: in as_sql
    extra_select, order_by, group_by = self.pre_sql_setup()
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:54: in pre_sql_setup
    self.setup_query()
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:45: in setup_query
    self.select, self.klass_info, self.annotation_col_map = self.get_select()
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:219: in get_select
    cols = self.get_default_columns()
.venv/lib/python3.6/site-packages/django/db/models/sql/compiler.py:666: in get_default_columns
    column = field.get_col(alias)
.venv/lib/python3.6/site-packages/django/db/models/fields/related.py:981: in get_col
    output_field = self.target_field
.venv/lib/python3.6/site-packages/django/db/models/fields/related.py:878: in target_field
    return self.foreign_related_fields[0]
.venv/lib/python3.6/site-packages/django/db/models/fields/related.py:632: in foreign_related_fields
    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)
.venv/lib/python3.6/site-packages/django/db/models/fields/related.py:619: in related_fields
    self._related_fields = self.resolve_related_fields()
.venv/lib/python3.6/site-packages/django/db/models/fields/related.py:604: in resolve_related_fields
    raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
E   ValueError: Related model 'appname.ModelName' cannot be resolved
=========================== 1 error in 5.54 seconds ============================

The test file:

$ cat …/apps/users/test/test_graph_permissions_command.py
from django.test import TestCase


class TestGraphPermissionsCommand(TestCase):
    def test_should_resolve_a_permission_hierarchy(self) -> None:
        pass

The circumstances:

Testing test_graph_permissions_command.py with another file works fine - any other file seems to work, and it works whether test_graph_permissions_command.py is first or last.
Importing TestCase from unittest instead works, but I will need some things from django.test.TestCase in the actual test.
Testing any other single file also works.
I rebuild the VM that this runs on every night, and a colleague was able to reproduce this issue, so it's not unique to my setup.
./manage.py makemigrations reports "No changes detected".
Many other test classes inherit from the same TestCase class without issue.
The app and model in the error message is very well tested. Ditto for the two models which each have a ForeignKey to "appname.ModelName". The code coverage is 98%.

What is going on here? It looks like this could be caused by a failure of the test DB builder to do things in the right sequence.

Django test setup fails when running single test file

0 Answers