A method model.objects.get(pk=obj.pk) returns many objects, but in a database is one.

[Seti]

Very strange behaviour of the Django ORM while testing (now I am using pytest)

I have two apps "books" and "replies".
The app "books" has two models ​http://pastebin.com/gDMi7UpU
The app "replies" has one model ​http://pastebin.com/u9FabQQA

For create new objects I am using factories for replies - ​http://pastebin.com/pGtiwS2m, for books and writers - ​http://pastebin.com/NQ6AWxib

While testing methods of queryset of the model Book I got error, where it should not be

A file querysets.py of the app "books" ​http://pastebin.com/FVnLLzxK
Tests for the file querysets.py of the app "books" ​http://pastebin.com/rmpDm0Ae

For testing I am using the pytest with next configuration in a file pytest.ini

[pytest]
addopts = -s --reuse-db --ff --maxfail=1 apps/books/tests/test_querysets.py::Tests
DJANGO_SETTINGS_MODULE=config.settings.development

Run test as next py.test

Traceback ​http://pastebin.com/Vv96XLzm

After many time googling and dipping in the Django`s code by the traceback I decided it is bug in the Django

I think it is the problem related with that models the Book and the Reply is related by help the field GenericRelation, but I don`t found facts for it.

Sorry for my grammar mistakes, I was no originally an English

If need another codes from my project, please tell about it

Update
Same error I had when tested a method "get_admin_url()" of the model Book with next a testing code

def test_get_admin_url(self):
        self.client.force_login(self.active_superuser)
        response = self.client.get(self.book.get_admin_url())
        self.assertEqual(response.status_code, 200)

Ending a traceback

        if not num:
            raise self.model.DoesNotExist(
                "%s matching query does not exist." %
                self.model._meta.object_name
            )
        raise self.model.MultipleObjectsReturned(
            "get() returned more than one %s -- it returned %s!" %
>           (self.model._meta.object_name, num)
        )
E       apps.books.models.MultipleObjectsReturned: get() returned more than one Book -- it returned 3!