使用线程时Django中的数据库错误

Question

我正在一个Django web应用程序中工作，它需要查询PostgreSQL数据库。当使用Pythonthreading接口实现并发时，我得到了查询项的DoesNotExist错误。当然，这些错误在按顺序执行查询时不会发生。在

让我展示一个单元测试，它是我用来演示意外行为的：

class ThreadingTest(TestCase):
    fixtures = ['demo_city',]

    def test_sequential_requests(self):
        """
        A very simple request to database, made sequentially.

        A fixture for the cities has been loaded above. It is supposed to be
        six cities in the testing database now. We will made a request for
        each one of the cities sequentially.
        """
        for number in range(1, 7):
            c = City.objects.get(pk=number)
            self.assertEqual(c.pk, number)

    def test_threaded_requests(self):
        """
        Now, to test the threaded behavior, we will spawn a thread for
        retrieving each city from the database.
        """

        threads = []
        cities = []

        def do_requests(number):
            cities.append(City.objects.get(pk=number))

        [threads.append(threading.Thread(target=do_requests, args=(n,))) for n in range(1, 7)]

        [t.start() for t in threads]
        [t.join() for t in threads]

        self.assertNotEqual(cities, [])

如您所见，第一个测试按顺序执行一些数据库请求，这些请求确实没有问题。然而，第二个测试执行完全相同的请求，但是每个请求都在一个线程中生成。这实际上是失败的，返回一个DoesNotExist异常。在

执行此单元测试的输出如下所示：

^{pr2}$

。。。其他线程返回类似的输出。。。

Exception in thread Thread-6:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 45, in do_requests
    cities.append(City.objects.get(pk=number))
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: City matching query does not exist.


FAIL

======================================================================
FAIL: test_threaded_requests (cesta.core.tests.threadbase.ThreadingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 52, in test_threaded_requests
    self.assertNotEqual(cities, [])
AssertionError: [] == []

----------------------------------------------------------------------
Ran 2 tests in 0.278s

FAILED (failures=1)
Destroying test database for alias 'default' ('test_cesta')...

请记住，所有这些都发生在PostgreSQL数据库中，它应该是线程安全的，而不是使用SQLite或类似的数据库。测试也是使用PostgreSQL运行的。在

在这一点上，我完全不知道什么会失败。有什么想法或建议吗？在

谢谢！在

编辑：我写了一个小视图来检查它是否能通过测试。以下是视图代码：

def get_cities(request):
    queue = Queue.Queue()

    def get_async_cities(q, n):
        city = City.objects.get(pk=n)
        q.put(city)

    threads = [threading.Thread(target=get_async_cities, args=(queue, number)) for number in range(1, 5)]

    [t.start() for t in threads]
    [t.join() for t in threads]

    cities = list()

    while not queue.empty():
        cities.append(queue.get())

    return render_to_response('async/cities.html', {'cities': cities},
        context_instance=RequestContext(request))

（请不要考虑应用程序的愚蠢之处。请记住，这只是一个概念的证明，不会永远在真正的应用程序。）

结果是代码运行良好，请求在线程中成功发出，并且视图在调用其URL后最终显示城市。在

所以，我认为只有在需要测试代码时，使用线程进行查询才是一个问题。在生产中，它将毫无问题地工作。在

对于成功测试这类代码有什么有用的建议吗？在

Answer 1

这听起来像是交易的问题。如果您在当前请求（或测试）中创建元素，那么它们几乎肯定是在一个未提交的事务中，该事务无法从另一个线程中的单独连接访问。您可能需要manage your transctions manually才能使其工作。在

Answer 2

从文档的这一部分变得更加清晰

class LiveServerTestCase(TransactionTestCase):
    """
    ...
    Note that it inherits from TransactionTestCase instead of TestCase because
    the threads do not share the same transactions (unless if using in-memory
    sqlite) and each thread needs to commit all their transactions so that the
    other thread can see the changes.
    """

现在，事务还没有在TestCase中提交，因此更改对另一个线程不可见。在

Answer 3

尝试使用TransactionTestCase：

class ThreadingTest(TransactionTestCase):

TestCase将数据保存在内存中，不向数据库发出提交。可能线程正在尝试直接连接到数据库，而数据还没有提交到那里。请参阅此处的说明： https://docs.djangoproject.com/en/dev/topics/testing/?from=olddocs#django.test.TransactionTestCase

TransactionTestCase and TestCase are identical except for the manner in which the database is reset to a known state and the ability for test code to test the effects of commit and rollback. A TransactionTestCase resets the database before the test runs by truncating all tables and reloading initial data. A TransactionTestCase may call commit and rollback and observe the effects of these calls on the database.