Django Rest Framework: Test serializers

In my project coach I have a problem with testing the Information serializer. I have the following serializer classes in the file running/serializes.py:

class Velocity(serializers.ModelSerializer):
    class Meta:
        model = VelocityModel
        fields = ("id", "minimum", "average", "maximum")


class Information(serializers.ModelSerializer):
    heart_beat = HeartBeat(read_only=True)
    velocity = Velocity(read_only=True)

    class Meta:
        model = InformationModel
        fields = ("id", "distance", "velocity", "heart_beat", "calories")

In my test I have this:

from running import models, serializers

@patch("running.serializers.Velocity")
def test_contains_id(self, mock_velocity):
    # mocking stuff
    returned_data = {}
    mock_velocity.data = PropertyMock(return_value=returned_data)

    # creating instances of the models
    self.velocity = models.Velocity(minimum=8, average=10, maximum=12)
    self.velocity.save()
    self.heart_beat = models.HeartBeat(minimum=120, average=130, maximum=140)
    self.heart_beat.save()
    self.information = models.Information(distance=3.7, velocity=self.velocity, heart_beat=self.heart_beat, calories=132)
    self.information.save()

    # create the actual serializer
    self.information_serializer = serializers.Information(self.information)

    self.assertEqual(self.information_serializer.data["velocity"], returned_data)

So I want to test, that the data returned by the InformationSerializer (self.information_serializer.data), has a key "velocity", which points on the data returned by the VelocitySerializer (mock_velocity.data).

But self.information_serializer.data["velocity"] just contains the data, saved in the models (OrderedDict([('id', 1), ('minimum', 8.0), ('average', 10.0), ('maximum', 12.0)]). I don't know where is my fault...

Also another question would be, do I really need to test this? Because I'm questioning myself, if I'm testing more the Django Rest Framework than my serializers?!

So how to go on? Thanks in advance!