Django Rest: how to mock FileField and ImageField to_representation()

I have a model with 2 fields: image (ImageField) and media (FileField). They both use a custom GoogleCloudMediaStorage.

When writing my tests, I cannot access GCS, so I mocked the writing of the files by doing this:

from unittest.mock import patch

    def test(self, mock_save):
        mock_save.return_value = 'mocked_file.png'
        # rest of the test

And it's working fine. The problem is now I have to test update and list views. When I do so, I have of course this error:

google.auth.exceptions.DefaultCredentialsError: Could not automatically determine credentials. Please set GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and re-run the application.

After digging it a bit, I see that the error is triggered by the serializer to_representation(). And more precisely in the FileField:

    def to_representation(self, value):
        if not value:
            return None

        use_url = getattr(self, 'use_url', api_settings.UPLOADED_FILES_USE_URL)
        if use_url:
                url = value.url #  !! THIS LINE CAUSE THE ISSUE
            except AttributeError:
                return None
            request = self.context.get('request', None)
            if request is not None:
                return request.build_absolute_uri(url)
            return url

So I tried to mock FileField.to_representation() doing this:

    def test_update(self, mock_representation):
        mock_representation.return_value = 'mocked_training_file_url'
        data = {
            'title': 'Updated',

        response = self.client.patch(. . .)

But I have this error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/unittest/", line 1322, in patched
    with self.decoration_helper(patched,
  File "/usr/local/lib/python3.8/", line 113, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.8/unittest/", line 1304, in decoration_helper
    arg = exit_stack.enter_context(patching)
  File "/usr/local/lib/python3.8/", line 425, in enter_context
    result = _cm_type.__enter__(cm)
  File "/usr/local/lib/python3.8/unittest/", line 1393, in __enter__
    original, local = self.get_original()
  File "/usr/local/lib/python3.8/unittest/", line 1366, in get_original
    raise AttributeError(
AttributeError: <class 'django.db.models.fields.files.FileField'> does not have the attribute 'to_representation'

Any idea how I can mock FileField and ImageField to_representation() ? Am I doing this correctly

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