text: from unittest import mock
  
  from django.db import connections
  from django.test import TestCase, TransactionTestCase, override_settings
  
  from .models import Car
  
  
  class TestSerializedRollbackInhibitsPostMigrate(TransactionTestCase):
      """
      TransactionTestCase._fixture_teardown() inhibits the post_migrate signal
      for test classes with serialized_rollback=True.
      """
  
      available_apps = ["test_utils"]
      serialized_rollback = True
  
      def setUp(self):
          # self.available_apps must be None to test the serialized_rollback
          # condition.
          self.available_apps = None
  
      def tearDown(self):
          self.available_apps = ["test_utils"]
  
      @mock.patch("django.test.testcases.call_command")
      def test(self, call_command):
          # with a mocked call_command(), this doesn't have any effect.
          self._fixture_teardown()
          call_command.assert_called_with(
              "flush",
              interactive=False,
              allow_cascade=False,
              reset_sequences=False,
              inhibit_post_migrate=True,
              database="default",
              verbosity=0,
          )
  
  
  @override_settings(DEBUG=True)  # Enable query logging for test_queries_cleared
  class TransactionTestCaseDatabasesTests(TestCase):
      available_apps = []
      databases = {"default", "other"}
  
      def test_queries_cleared(self):
          """
          TransactionTestCase._pre_setup() clears the connections' queries_log
          so that it's less likely to overflow. An overflow causes
          assertNumQueries() to fail.
          """
          for alias in self.databases:
              self.assertEqual(
                  len(connections[alias].queries_log), 0, "Failed for alias %s" % alias
              )
  
  
  class DisallowedDatabaseQueriesTests(TransactionTestCase):
      available_apps = ["test_utils"]
  
      def test_disallowed_database_queries(self):
          message = (
              "Database queries to 'other' are not allowed in this test. "
              "Add 'other' to test_utils.test_transactiontestcase."
              "DisallowedDatabaseQueriesTests.databases to ensure proper test "
              "isolation and silence this failure."
          )
          with self.assertRaisesMessage(AssertionError, message):
              Car.objects.using("other").get()
  