[Django]-Unittest Django: Mock external API, what is proper way?

You should mock the direct collaborators of the code under test. For Book that would be utils. For utils that would be requests.

So for the BookModelTestCase:

class BookModelTestCase(TestCase):

    @mock.patch('app.models.utils')
    def test_save_book_calls_google_api(self, mock_utils):
        mock_utils.get_original_title_and_name.return_value = {
            'title': 'Google title',
            'author': 'Google author'
        }

        book = Book(
            title='Some title',
            isbn='12345'
        )
        book.save()

        self.assertEqual(book.title, 'Google title')
        self.assertEqual(book.author, 'Google author')
        mock_utils.get_original_title_and_name.assert_called_once_with('12345')

And then you can create a separate test case to test get_original_title_and_name:

class GetOriginalTitleAndNameTestCase(TestCase):

    @mock.patch('app.utils.requests.get')
    def test_get_original_title_and_name_from_google_api(self, mock_get):
        mock_response = mock.Mock()
        # Define response data from Google API
        expected_dict = {
            'kind': 'books#volumes',
            'totalItems': 1,
            'items': [
                {
                    'kind': 'books#volume',
                    'id': 'IHxXBAAAQBAJ',
                    'etag': 'B3N9X8vAMWg',
                    'selfLink': 'https://www.googleapis.com/books/v1/volumes/IHxXBAAAQBAJ',
                    'volumeInfo': {
                        'title': "Alice's Adventures in Wonderland",
                        'authors': [
                            'Lewis Carroll'
                        ]
                    }
                }
                    ]
            }

        # Define response data for my Mock object
        mock_response.json.return_value = expected_dict
        mock_response.status_code = 200

        # Define response for the fake API
        mock_get.return_value = mock_response

        # Call the function
        result = get_original_title_and_name(12345)

        self.assertEqual(result, {
            'title': "Alice's Adventures in Wonderland", 
            'author': 'Lewis Carroll'
        })
        mock_get.assert_called_once_with(GOOGLE_API_URL, params={
            'key': GOOGLE_API_KEY,
            'q': 'isbn:12345',
            'printType': 'books',
        })