Django test resolves by for loop

Question

I am doing some test in Django. I use fixtures to pre load some data and test API POST. In my POST, I want to link some data by foreign key. The test behavior is odd. I need to place a for loop in order for it to succeed.

Here's my example for my fixtures:

[
  {
    "model": "apps.track",
    "pk": 1,
    "fields": {
      "name": "Love Song # 1",
      "album": null
    }
  },
  {
    "model": "apps.track",
    "pk": 2,
    "fields": {
      "name": "Love Song # 2",
      "album": null
    }
  }
]

Here's my test:

class TestPost(TestCase):
    fixtures = ['fixtures/sample']

    def setUp(self):
        self.album = {
            'name': 'Love Songs Album',
            'tracks': [
                {'id': 1},
                {'id': 2}
            ]
        }

    def test_post(self):
        c = Client()
        response = c.post(
            '/admin/login/', {'username': 'admin', 'password': 'passwordpassword'})
        response = c.post('/apps/albums/',
                          data=json.dumps(self.album),
                          content_type='application/json')

        album = Album.objects.latest('id')
        tracks = Track.objects.filter(album=album).all()

        self.assertEqual(tracks[0].id, 1)
        self.assertEqual(tracks[1].id, 2)

The test fails and only finds tracks id=2 but not tracks id=1. However when I add a for loop before the test assertions, the test succeeds.

Here's the magic for loop:

for track in tracks:
    print("TRACK", track.id)

I want to know if there is any reason behind this.