Django REST Framework: return 404 (not 400) on POST if related field does not exist?

Question

I'm developing a REST API which takes POST requests from some really brain-dead software which can't PATCH or anything else. The POSTs are to update Model objects which already exist in the database.

Specifically, I'm POSTing data for objects with a related field (a SlugRelatedField, as the POSTer knows the 'name' attribute but NOT the 'pk'). However, I need to return a 404 if the POSTer sends data where the 'name' returns nothing on the SlugRelatedField (e.g. the related object does not exist). I've been through this with a debugger but it seems that DRF uses some Django signals magic to do it The Way DRF Does It™, which is to return a 400 BAD REQUEST. I don't know how to modify this - only when it's the above condition and not a true 400-worthy POST - into a 404.

By the way, pre_save() in my view is NOT executing during execution of the failing test.

Here's the serializer:

class CharacterizationSerializer(serializers.ModelSerializer):
    """
    Work-in-progress for django-rest-framework use.  This handles (de)serialization
    of data into a Characterization object and vice versa.

    See: http://www.django-rest-framework.org/tutorial/1-serialization
    """
    creator = serializers.Field(source='owner.user.username')
    sample = serializers.SlugRelatedField(slug_field='name',
                                          required=True,
                                          many=False,
                                          read_only=False)

    class Meta:
        model = Characterization
        # leaving 'request' out because it's been decided to deprecate it. (...maybe?)
        fields = ('sample', 'date', 'creator', 'comments', 'star_volume', 'solvent_volume',
                  'solution_center', 'solution_var', 'solution_minimum', 'solution_min_stddev',
                  'solution_test_len',)

And here's the view where pre_save isn't being run in the given test (but does get run in some others):

class CharacterizationList(generics.ListCreateAPIView):
    queryset = Characterization.objects.all()
    serializer_class = CharacterizationSerializer
    permission_classes = (AnonPostAllowed,)   # @todo XXX hack for braindead POSTer

    def pre_save(self, obj):
        # user isn't sent as part of the serialized representation,
        # but is instead a property of the incoming request.
        if not self.request.user.is_authenticated():
            obj.owner = get_dummy_proxyuser()   # this is done for CharacterizationList so unauthed users can POST. @todo XXX hack
        else:
            obj.owner = ProxyUser.objects.get(pk=self.request.user.pk)

        # here, we're fed a string sample name, but we need to look up
        # the actual sample model.
        # @TODO: Are we failing properly if it doesn't exist?  Should
        # throw 404, not 400 or 5xx.
        # except, this code doesn't seem to be run directly when debugging.
        # a 400 is thrown; DRF must be bombing out before pre_save?
        obj.sample = Sample.objects.get(name=self.request.DATA['sample'])

And for good measure, here's the failing test:

def test_bad_post_single_missing_sample(self):
    url = reverse(self._POST_ONE_VIEW_NAME)

    my_sample_postdict = self.dummy_plqy_postdict.copy()
    my_sample_postdict["sample"] = "I_DONT_EXIST_LUL"
    response = self.rest_client.post(url, my_sample_postdict)
    self.assertTrue(response.status_code == 404,
                    "Expected 404 status code, got %d (%s). Content: %s" % (response.status_code, response.reason_phrase, response.content))

If I put a breakpoint in at the self.rest_client.post() call, the response already has a 400 in it at that point.

Answer 1

You can use a Django Shortcut for that, getting the obj.sample:

from django.shortcuts import get_object_or_404
obj.sample = get_object_or_404(Sample, name=self.request.DATA['sample'])