Getting non_field_errors using JSONField ()

Question

I am trying to make a PATCH request using the Django Rest Framework, but getting the following error:

{"image_data": [{"non_field_errors": ["Invalid data"]}] 

I understand that JSONField () can give some problems, so I took care of this by adding to_native and from_native . But I am still facing this problem. I don’t think the JSONField() problem is here at all, but still worth mentioning.

I believe that I am doing something fundamentally wrong in the way I try to update a related area.

Code below ...

Model:

 class Photo(models.Model): user = models.ForeignKey(AppUser, help_text="Item belongs to.") image_data = models.ForeignKey("PhotoData", null=True, blank=True) class PhotoData(models.Model): thisdata = JSONField() 

serializers:

 class ExternalJSONField(serializers.WritableField): def to_native(self, obj): return json.dumps(obj) def from_native(self, value): try: val = json.loads(value) except TypeError: raise serializers.ValidationError( "Could not load json <{}>".format(value) ) return val class PhotoDataSerializer(serializers.ModelSerializer): thisdata = ExternalJSONField() class Meta: model = PhotoData fields = ("id", "thisdata") class PhotoSerializer(serializers.ModelSerializer): image_data = PhotoDataSerializer() class Meta: model = Photo fields = ("id","user", "image_data") 

PATCH:

 > payload = {"image_data": {"thisdata": "{}"}} > requests.patch("/photo/123/",payload ) 

I also tried:

 > payload = {"image_data": [{"thisdata": "{}"}]} > requests.patch("/photo/123/",payload ) 

But again with the same error:

[{"non_field_errors": ["Invalid data"]}]

Answer 1

The initial idea for serializing Django Rest Framework relationships is to not change the values ​​of the related fields. This means that your payload should contain the pk object of the PhotoData object, not the dataset for it. As with models, you cannot assign a dict to a foreign key field.

Good (only works with serializers.PrimaryKeyRelatedField, which contains the problems themselves):

  payload = {"image_data": 2} 

Bad (does not work in DRF by default):

  payload = {"image_data": {'thisdata': '{}'}} 

In fact, the data model you provided does not need PhotoData at PhotoData (you can move the thisdata field to Photo ), but suppose you have a special case, even when Zen of Python says Special cases aren't special enough to break the rules. .

So, here are a few possible ways:

Using field serializers (your original way)

What you want to do now is a possible but very ugly decision. You can create a PhotoDataField (works for me, but not ready to use code, just for demonstration)

 class PhotoDataField(serializers.PrimaryKeyRelatedField): def field_to_native(self, *args): """ Use field_to_native from RelatedField for correct `to_native` result """ return super(serializers.RelatedField, self).field_to_native(*args) # Prepare value to output def to_native(self, obj): if isinstance(obj, PhotoData): return obj.thisdata return super(PhotoDataField, self).to_native(obj) # Handle input value def field_from_native(self, data, files, field_name, into): try: int(data['image_data']) except ValueError: # Looks like we have a data for `thisdata` field here. # So let do write this to PhotoData model right now. # Why? Because you can't do anything with `image_data` in further. if not self.root.object.image_data: # Create a new `PhotoData` instance and use it. self.root.object.image_data = PhotoData.objects.create() self.root.object.image_data.thisdata = data['image_data'] self.root.object.image_data.save() return data['image_data'] except KeyError: pass # So native behaviour works (eg via web GUI) return super(PhotoDataField, self).field_from_native(data, files, field_name, into) 

and use it in PhotoSerializer

 class PhotoSerializer(serializers.ModelSerializer): image_data = PhotoDataField(read_only=False, source='image_data') class Meta: model = Photo fields = ("id", "user", "image_data") 

so the request will work well

 payload = {"image_data": '{}'} resp = requests.patch(request.build_absolute_uri("/api/photo/1/"), payload) 

and a "good" request also

 photodata = PhotoData.objects.get(pk=1) payload = {"image_data": photodata.pk} resp = requests.patch(request.build_absolute_uri("/api/photo/1/"), payload) 

and as a result you will see in the GET request "image_data": <photodata thisdata value>,

But even if you fix the validation problems with this approach, it will still be a huge pain in the ass, as you can see from my code (this is just what DRF can offer you when you want to “disrupt the normal workflow”, Tastypie offers more )

Normalize code and use @action (recommended)

 class PhotoDataSerializer(serializers.ModelSerializer): class Meta: model = PhotoData fields = ("id", "thisdata") class PhotoSerializer(serializers.ModelSerializer): image_data = PhotoDataSerializer() # or serializers.RelatedField class Meta: model = Photo fields = ("id", "user", "image_data", "test") 

and now define a specific method in your api view that you can use to set the data for any photo

 from rest_framework import viewsets, routers, generics from rest_framework.decorators import action from rest_framework.response import Response from rest_framework import status # ViewSets define the view behavior. class PhotoViewSet(viewsets.ModelViewSet): model = Photo serializer_class = PhotoSerializer @action(methods=['PATCH']) def set_photodata(self, request, pk=None): photo = self.get_object() serializer = PhotoDataSerializer(data=request.DATA) if serializer.is_valid(): if not photo.image_data: photo.image_data = PhotoData.objects.create() photo.save() photo.image_data.thisdata = serializer.data photo.image_data.save() return Response({'status': 'ok'}) else: return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST) 

Now you can make almost the same request as now, but you have much more extensibility and separation of duties in the code. See URL , it is added when you have a method processed by @action.

 payload = {"thisdata": '{"test": "ok"}'} resp = requests.patch(request.build_absolute_uri("/api/photo/1/set_photodata/"), payload) 

Hope this helps.