Django Testing error Object of type is not JSON serializable

Question

I am testing an api and have some trouble when testing for objects with ForeignKeys.

This is what a test that fails looks like:

test_models.py

class GameroundViewTests(APITestCase):

    def setUp(self):
        self.client = APIClient()
        self.user = CustomUser.objects.create(username="carina")
        self.gametype = Gametype.objects.create(name="NewGame", rounds=5, round_duration=60, enabled=True)
        self.gamesession = Gamesession.objects.create(user=self.user, gametype=self.gametype, created=datetime.now())
        self.gameround_data = {'gamesession': self.gamesession}
        self.response = self.client.get('http://localhost:8000/api/gameround',
                                        self.gameround_data,
                                        format="json")
    def test_get(self):
        self.client = APIClient()
        response = self.client.get('http://localhost:8000/api/gameround')
        self.assertEqual(response.status_code, 200)
        # self.assertEqual(len(self.response.data), len(Gameround.objects.all().order_by('?').first()))

    def test_post(self):
        self.client = APIClient()
        # response = self.client.get('http://localhost:8000/api/gameround')
        data = {
            'id': 1,
            'user': self.user,
            'gamesession': self.gamesession,
            'created': datetime.now(),
            'score': 0,
        }
        response = self.client.post('http://localhost:8000/api/gameround', data=data, format='json')
        self.assertEqual(response.status_code, 201)

I am getting the error: GameroundViewTests::test_post - TypeError: Object of type CustomUser is not JSON serializable

If I change my test_post to:

    def test_post(self):
        self.client = APIClient()
        self.client.get('http://localhost:8000/artigo_api/gameround')
        user = {'username': self.user.id}
        gamesession = {"gamesession": self.gamesession.id}
        data = {
            'id': 1,
            'user': user,
            'gamesession': gamesession,
            'created': datetime.now(),
            'score': 0,
        }
        self.assertEqual(Gameround.objects.count(), 0)
        response = self.client.post('http://localhost:8000/artigo_api/gameround', data=data, format='json')
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        self.assertEqual(Gameround.objects.count(), 1)
        gameround = Gameround.objects.all().first()
        for field_name in data.keys():
            self.assertEqual(getattr(gameround, field_name), data[field_name])

I am getting the error: GameroundViewTests::test_post - KeyError: 'gamesessions'

What can I do to get rid of this and for the test_post to pass?