Django,TestCase,Thread

バージョン
python 3.6.4
django 2.0.3

以下のテストは成功するような気がするけど失敗する

class Test(TestCase):
    def threadmain(self):
        self.assertTrue(Foo.objects.all().count()>0)

    def test(self):
        #オブジェクトを作成する
        Foo.objects.create()

        th = Thread(target=self.threadmain)
        th.start()
        th.join()

        self.assertTrue(Foo.objects.all().count()>0)

結果

Creating test database for alias 'default'...
System check identified no issues (0 silenced).
........................................................Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/local/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/mnt/c/prj/sao/sao/tests.py", line 1414, in threadmain
    self.assertTrue(Foo.objects.all().count()>0)
  File "/usr/local/lib/python3.6/unittest/case.py", line 682, in assertTrue
    raise self.failureException(msg)
AssertionError: False is not true

.....................................

Ran 93 tests in 3.652s

OK
Destroying test database for alias 'default'...

対処
TransactionTestCaseを継承する

class Test(TransactionTestCase):
    def threadmain(self):
        self.assertTrue(Foo.objects.all().count()>0)

    def test(self):
        #オブジェクトを作成する
        Foo.objects.create()

        th = Thread(target=self.threadmain)
        th.start()
        th.join()

        self.assertTrue(Foo.objects.all().count()>0)

結果

Creating test database for alias 'default'...
System check identified no issues (0 silenced).

.............................................................................................

Ran 93 tests in 4.101s

OK
Destroying test database for alias 'default'...