Use get_serializer_class in ordering filter

[kmwenja]

OrderingFilter backend checks whether a view specifies an
ordering_fields or serializer_class attribute. Views can however
override get_serializer_class to determine the serializer class
but the OrderingFilter doesn't catch this. This patch fixes that.

Here's an example of a view that would benefit from this patch:

class MyView(generics.ListAPIView):
    queryset = MyModel.objects.all()
    filter_backends = (filters.OrderingFilter, )

    def get_serializer_class(self):
        return MySerializer

This would formerly raise an ImproperlyConfigured error.

[xordoquy]

Do you know why it doesn't ?

[kmwenja]

@xordoquy Sorry, I realized that the get_serializer_class method actually has its own assert to check for serializer_class: https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/generics.py#L123.

On that note, is it better to get the error from get_serializer_class (AssertionError) or from remove_invalid_fields (ImproperlyConfigured)? I can rewrite the patch to either catch the AssertionError in get_serializer_class or let it pass right up.

[kmwenja]

@xordoquy I preferred raising ImproperlyConfigured (since that was the initial intended purpose) but I can quickly change that.

[kmwenja]

@xordoquy would it be presumptuous to update the docs at this point?

[xordoquy]

We'll take some time to review it first.

[codecov-io]

Current coverage is 91.43%

Merging #3487 into master will increase coverage by <.01%

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+ Misses          471     459    -12   
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[kmwenja]

@xordoquy Updated the PR against master. Apologies for the lack of love.

[tomchristie]

Perfectly reasonable, yup. Thanks!