add read_only check for dotted-source fields in serializers

[leahoswald]

The error message tells that you should create an own crate() function
or set read_only=True for the serializer field. The Problem is that the check
ignores this read_only flag.

[tomchristie]

We check 'key in validated_data' so we are checking if it's being written to. Closing this off given that, but happy to reopen if you can explain the motivation further in light of that. Eg do you have an example failure case. Also note #2266 which went into master today, and improves the robustness of the check (may be relevant to whatever you're seeing?)

[leahoswald]

It looks like #2266 fixed the problem for me. It happened in an usecase similar to this the one described in the Tutorial http://www.django-rest-framework.org/tutorial/4-authentication-and-permissions/#associating-snippets-with-users

Model:

class CategoryViewSet(NestedViewSetMixin, viewsets.ModelViewSet):               
      queryset = Category.objects.all()                                           
      serializer_class = CategorySerializer                                       
      def perform_create(self, serializer):                                       
         serializer.save(user = self.request.user) 

Serializer:

class CategorySerializer(serializers.ModelSerializer):                          
     user = serializers.ReadOnlyField( source='user.id')          
     class Meta:                                                                 
          model = Category                                                        
          fields = ('id', 'user', 'title')

This was the traceback: http://dpaste.com/0FT2MY1

[tomchristie]

Great. Probably worth us rolling 3.0.2 reasonably soon then.