TransactionManagementError "You can't execute queries until the end of the 'atomic' block" while using signals, but only during Unit Testing

I ran into this same problem myself. This is caused by a quirk in how transactions are handled in the newer versions of Django coupled with a unittest that intentionally triggers an exception.

I had a unittest that checked to make sure a unique column constraint was enforced by purposefully triggering an IntegrityError exception:

def test_constraint(self):
    try:
        # Duplicates should be prevented.
        models.Question.objects.create(domain=self.domain, slug='barks')
        self.fail('Duplicate question allowed.')
    except IntegrityError:
        pass

    do_more_model_stuff()

In Django 1.4, this works fine. However, in Django 1.5/1.6, each test is wrapped in a transaction, so if an exception occurs, it breaks the transaction until you explicitly roll it back. Therefore, any further ORM operations in that transaction, such as my do_more_model_stuff(), will fail with that django.db.transaction.TransactionManagementError exception.

Like caio mentioned in the comments, the solution is to capture your exception with transaction.atomic like:

from django.db import transaction
def test_constraint(self):
    try:
        # Duplicates should be prevented.
        with transaction.atomic():
            models.Question.objects.create(domain=self.domain, slug='barks')
        self.fail('Duplicate question allowed.')
    except IntegrityError:
        pass

That will prevent the purposefully-thrown exception from breaking the entire unittest's transaction.

Since @mkoistinen never made his comment, an answer, I'll post his suggestion so people won't have to dig through comments.

consider just declaring your test class as a TransactionTestCase rather than just TestCase.

From the Django docs: A TransactionTestCase may call commit and rollback and observe the effects of these calls on the database.

If using pytest-django you can pass transaction=True to the django_db decorator to avoid this error.

See https://pytest-django.readthedocs.io/en/latest/database.html#testing-transactions

Django itself has the TransactionTestCase which allows you to test transactions and will flush the database between tests to isolate them. The downside of this is that these tests are much slower to set up due to the required flushing of the database. pytest-django also supports this style of tests, which you can select using an argument to the django_db mark:

@pytest.mark.django_db(transaction=True)
def test_spam():
    pass  # test relying on transactions