How can one return 400 status code with DRF APIView get method and ModelSerializer? - validation

Question

I have the following code (for the endpoint /things/{id}/permission-to-do/
views.py
class PermissionsToDo(APIView):
authentication_classes = (TokenAuthentication,)
permission_classes = (IsAuthenticated,)
def get(self, request,*args,**kwargs):
thing_id = kwargs.get('pk')
thing = Thing.objects.filter(pk=thing_id,is_active=True)
serializer = serializers.GetDoPermissionSerializer(thing[0],context={'request': request})
return Response(serializer.data, status=status.HTTP_200_OK)
serializers.py
class serializers.GetDoPermissionSerializer(serializers.ModelSerializer):
def _can_do(self, thing):
return thing.can_be_done_by(self.context['request'].user)
can_do = serializers.SerializerMethodField('_can_do')
class Meta:
model = Thing.objects.filter
fields = ('can_do',)
extra_kwargs = {
'can_do': {'read_only': True},
}
The thing.can_be_done_by(user) method returns a Boolean. This works fine with a correct request but I want to add a way to validate the request and send appropriate status code for client errors, such as status.HTTP_400_BAD_REQUEST
My idea was to just add in views.py a:
if serializer.is_valid():
return Response(serializer.data, status=status.HTTP_200_OK)
else:
return Response(SOMETHING?,status.HTTP_400_BAD_REQUEST)
if I evaluate serializer.is_valid() I obtain an error message saying:
'Cannot call `.is_valid()` as no `data=` keyword argument was '
AssertionError: Cannot call `.is_valid()` as no `data=` keyword argument was passed when instantiating the serializer instance.
if I change the line:
serializer = serializers.GetDoPermissionSerializer(thing[0],context={'request': request})
into:
serializer = serializers.GetDoPermissionSerializer(data=thing[0],context={'request': request})
but then, I get an error suggesting the I should be passing a dictionary as data and not an object. but then I'm not sure how to implement the validate method and how to change the _can_do method to get it to work.
Any idea?
Thanks for your time if you have some to spare!

Answer 1

.validate() method takes a single argument, which is a dictionary of field values. It should raise a ValidationError if necessary, or just return the validated values.
One of the ways you can achieve this is to convert the model object to a dict. So try the following snippet,
class PermissionsToDo(APIView):
authentication_classes = (TokenAuthentication,)
permission_classes = (IsAuthenticated,)
def get(self, request, *args, **kwargs):
thing_id = kwargs.get('pk')
thing = Thing.objects.filter(pk=thing_id, is_active=True)
serializer = serializers.GetDoPermissionSerializer(data=thing[0].__dict__, context={'request': request}) # Change is here <<<<
if serializer.is_valid():
return Response(serializer.data, status=status.HTTP_200_OK)
return Response(SOMETHING?, status.HTTP_400_BAD_REQUEST)
My suggestion
If you are serializing the object from DB, most of the time it won't raise any validation error. I would suggest that, try to show error message if thing object become a empty queryset.So,
class PermissionsToDo(APIView):
authentication_classes = (TokenAuthentication,)
permission_classes = (IsAuthenticated,)
def get(self, request, *args, **kwargs):
thing_id = kwargs.get('pk')
try:
serializer = serializers.GetDoPermissionSerializer(Thing.objects.get(id=thing_id), context={'request': request})
return Response(serializer.data, status=status.HTTP_200_OK)
except Thing.DoesNotExist:
return Response(data="object not found", status.HTTP_400_BAD_REQUEST)

Answer 2

I used your "suggestion" to the exception that I had to modify the last line. I replaced:
return Response(data="object not found", status.HTTP_400_BAD_REQUEST)
by :
return Response("object not found", status.HTTP_400_BAD_REQUEST)
or I get the following error:
SyntaxError: positional argument follows keyword argument