Django REST Framework: how serializer context works?

Question

It's a pretty standard task in Django REST Framework to supply additional args/kwargs to a serializer to set values of fields set not via request.data, but via the value in url parameters or cookies. For instance, I need to set user field of my Comment model equal to request.user upon POST request. Those additional arguments are called context.

Several questions (1, 2) on StackOverflow suggest that I override get_serializer_context() method of my ModelViewSet. I did and it doesn't help. I tried to understand, what's wrong, and found out that I don't understand from the source code, how this context system is supposed to work in general. (documentation on this matter is missing, too)

Can anyone explain, where serializer adds context to normal request data? I found two places, where it saves the values from context.

1. serializer.save(), method, which mixes kwargs with validated data, but it is usually called with no arguments (e.g. by ModelMixins).
2. fields.__new__(), which caches args and kwargs, but it seems that nobody ever reads them later.

Answer 1

Whenever you use generic views or viewsets, DRF(3.3.2) adds request object, view object and format to the serializer context. You can use serializer.context to access, lets say request.user in the serializer.

This is added when get_serializer_class() is called. Inside that, it calls get_serializer_context() method where all these parameters are added to its context.

DRF source code for reference:

class GenericAPIView(views.APIView):
    """
    Base class for all other generic views.
    """

    def get_serializer(self, *args, **kwargs):
        """
        Return the serializer instance that should be used for validating and
        deserializing input, and for serializing output.
        """
        serializer_class = self.get_serializer_class()
        kwargs['context'] = self.get_serializer_context()
        return serializer_class(*args, **kwargs)    

    def get_serializer_context(self):
        """
        Extra context provided to the serializer class.
        """
        return {
            'request': self.request,
            'format': self.format_kwarg,
            'view': self
        }

Answer 2

to set values of fields set not via request.data, but via the value in url parameters or cookies. For instance, I need to set user field of my Comment model equal to request.user upon POST request.

This is how I handle both cases in my ModelViewSet:

def perform_create(self, serializer):

    # Get article id from url e.g. http://myhost/article/1/comments/
    # obviously assumes urls.py is setup right etc etc
    article_pk = self.kwargs['article_pk']
    article = get_object_or_404(Article.objects.all(), pk=article_pk)

    # Get user from request
    serializer.save(author=self.request.user, article=article)

Unfortunately the nested objects is not standard for DRF but that's besides the point. :)