DRF Viewset - Return a 400 if object exists and a 200 if it doesn't

Question

Using Django Rest Framework, I'm trying to create an API endpoint to determine if a given name of an item, already exists.

If it does, then we should let the frontend app know by returning a 400 Bad Request. If it does not exists, then we send a positive 200 OK request.

I find that I can send a 400 just fine but if the item doesn't exist, I get a 404 that I can do nothign about.

views.py

from rest_framework import viewsets,status
from rest_framework.response import Response
from .serializers import *

class ItemCheckViewSet(viewsets.ModelViewSet):
    """
    The views that are returned when we peform checks against the items
    """
    lookup_field = 'name'
    queryset = Item.objects.all()

    def get_serializer_class(self):
        return ItemSerializer

    def retrieve(self, request, *args, **kwargs):
        instance = self.get_object()
        serializer = self.get_serializer(instance)

        if len(serializer.data) > 0:
            return Response("Item already exists", status.HTTP_400_BAD_REQUEST)

        return Response("Item does not exist", status.HTTP_200_OK)

serializers.py

from rest_framework import serializers
from .models import Item


class ItemSerializer(serializers.ModelSerializer):
    class Meta:
        model = Item
        fields = "__all__"

I think that this post can help you [DRF custom error](https://stackoverflow.com/questions/51665260/django-rest-framework-custom-error-message) — Rvector, Sep 02 '21 at 22:01

score 1 · Answer 1 · answered Sep 02 '21 at 22:16

You can do that with overriding get_object method. By default this method use get_object_or_404(queryset, **filter_kwargs). When this method can't find object, raises Http404 and return 404 status code. But you must watch out for other requestactions e.g post, put. This actions also ask about object the common get_object method

score 1 · Accepted Answer · answered Sep 02 '21 at 23:25

No need to serialize if the object exists, and you can just check if a Http404 got raised from get_object:

from django.http import Http404


class ItemCheckViewSet(viewsets.ModelViewSet):
    ...
    def retrieve(self, request, *args, **kwargs):
        try:
            instance = self.get_object()
        except Http404:
            return Response("Item does not exist", status.HTTP_200_OK)
        return Response("Item already exists", status.HTTP_400_BAD_REQUEST)

DRF Viewset - Return a 400 if object exists and a 200 if it doesn't

views.py

serializers.py

2 Answers2