Django unit test - How to assert that a custom Manager calls the UserManager.create_user method?

I'm a newcomer to unit tests and mocking in Python, and in Django too.

Here is my model (simplified):

class KangaUserManager(models.Manager):
    def create(self, username, email, password, last_name, first_name, request, registered=True, send_confirmation=True):
        kanga_user = KangaUser()
        kanga_user.user = User.objects.create_user(
            username=username,
            email=email,
            password=password,
            first_name=first_name,
            last_name=last_name
        )
        return kanga_user

class KangaUser(models.Model):
    objects = KangaUserManager()

    user = models.OneToOneField(User)

I would like to assert thet the User.objects.create_user is called, without actually calling it. I see that the mock library is the one to use, I think, but I don't get to anything.

I tried things like that :

def test_create(self):
    with mock.patch('django.contrib.auth.models.UserManager') as usermanager_mock:
        kangauser_manager = KangaUserManager()
        kangauser_manager.create(self.username, self.email, self.password, self.last_name, self.first_name, self.request, self.registered, self.send_confirmation)

        self.assertTrue(usermanager_mock.create_user.called)

and that (and many more things I can't remeber :D)

def test_create(self):
    with mock.patch('django.contrib.auth.models.User') as user_mock:
        user_mock.objects = mock.MagicMock()
        user_mock.objects.create_user = mock.MagicMock()

        kangauser_manager = KangaUserManager()
        kangauser_manager.create(self.username, self.email, self.password, self.last_name, self.first_name, self.request, self.registered, self.send_confirmation)

        self.assertTrue(user_mock.objects.create_user.called)

But I always end up seeing that the method is not called

What am I doing wrong ?