DRF-PostViewSerializerInjectUrlDataMixin.py

class InjectUrlDataMixin:
    """Used for POST on nested router (so it can take infromation from URL instead of repeting in serializer).
    overrides `get_serializer`
    (ps.: does not make sense for PUT/PATCH -- as the instance is selected by url-kwargs)
    """
    def get_serializer(self, *args, **kwargs):
        # standart `get_serializer` logic
        serializer_class = self.get_serializer_class()
        kwargs['context'] = self.get_serializer_context()

        # our logic
        if self.request.method == 'POST':
            # prepare/clone data container (QueryDict)
            data = kwargs.get('data', {}) # get data or create new {}
            data = data.copy() # make a copy (as better not to change `request.data` QueryDict)
            kwargs['data'] = data  # and assign back into kwargs

            # inject info from url-kwargs
            url_kwargs = self.kwargs  # this info also resides in kwargs['context']['view']
            for key, val in url_kwargs.items():  
                data.setdefault(key, val) # don't overrite if exists

        return serializer_class(*args, **kwargs)