Not able to find object from DB right after I created It - django

Question

I was testing User creation by creating a TestCase, and it couldn't be found right after I created it.
I've tried to flush the cache by calling .refresh_from_db(), but it doesn't work.
Here is my TestCase:
class SuperStrangeTest(TestCase):
def test_super_strange(self):
john = User.objects.create()
john.refresh_from_db()
print('!=====START' * 10)
print(User.objects.count())
print(User.objects.all())
self.assertIsNotNone(User.objects.filter().first()) # None of assertions below would be right
self.assertIsNotNone(User.objects.filter(id=john.id).first())
self.assertTrue(User.objects.filter(id=john.id).exists())
My command to run this test is:
./manage.py test --noinput --failfast --keepdb links.tests.SuperStrangeTest.test_super_strange
The result sometimes went right, but most times it is just broken.
Using existing test database for alias 'default'...
/Users/oldcai/.virtualenvs/web/lib/python3.7/site-packages/grequests.py:21: MonkeyPatchWarning: Patching more than once will result in the union of all True parameters being patched
curious_george.patch_all(thread=False, select=False)
System check identified no issues (0 silenced).
!=====START!=====START!=====START!=====START!=====START!=====START!=====START!=====START!=====START!=====START
1
<QuerySet [<User: >]>
F
======================================================================
FAIL: test_super_strange (links.tests.SuperStrangeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/oldcai/programs/python/webproject/zine/links/tests.py", line 41, in test_super_strange
self.assertIsNotNone(User.objects.filter().first())
AssertionError: unexpectedly None
----------------------------------------------------------------------
Ran 1 test in 0.130s
FAILED (failures=1)
Preserving test database for alias 'default'...
Errors of other lines:
======================================================================
FAIL: test_super_strange (links.tests.SuperStrangeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/oldcai/programs/python/webproject/links/tests.py", line 35, in test_super_strange
self.assertTrue(User.objects.filter(id=john.id).exists())
AssertionError: False is not true
----------------------------------------------------------------------
======================================================================
FAIL: test_super_strange (links.tests.SuperStrangeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/oldcai/programs/python/webproject/links/tests.py", line 35, in test_super_strange
self.assertTrue(User.objects.filter(id=john.id).exists())
AssertionError: False is not true
----------------------------------------------------------------------

Answer 1

Try to fill your model fields. It's base Django User model - it requires username and password. Also to create django user - use create_user function, that function will hash the password for you. Try to correct you code like that:
...
john = User.objects.create_user(username='john', password='password')
...

Answer 2

After diagnosed deeper into this issue, it appears that this error relates to my DATABASE_ROUTERS setting.
I'm routing the reading part of operations to a random read-only slave database in production to balancing the load of reading.
In TestCase, I set the configs of the slave database just equal to the default one.
DATABASE = {
'ENGINE': 'django.db.backends.postgresql',
'ATOMIC_REQUESTS': False,
'CONN_MAX_AGE': 0,
'NAME': 'test',
'USER': 'test',
'PASSWORD': 'test',
'HOST': '',
'PORT': '',
}
DATABASES = {
'default': DATABASE,
'replica1': DATABASE,
}
But it still not able to query out the result with replica1 right after a record inserted by default database.
When the router randomly chose the default database as the one to read from, the TestCase would pass, otherwise, it would fail.