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

Question

I am getting TransactionManagementError when trying to save a Django User model instance and in its post_save signal, I\'m saving some models that have the user as the forei

Answer 1

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.