Django REST Framework Serializer validation error

Question

I have the following serializer:

class SearchSerializer(serializers.Serializer):
    category_id = serializers.IntegerField()
    search_term = serializers.CharField(required=False)

I need to validate whether the category with the id exists in the database and retrieve it's value in the view. In order to avoid fetching Category twice, I have added the following validate() to the serializer.

def validate(self, data):
     try:
         category = Category.objects.get(pk=data['category_id'])
     except Category.DoesNotExist:
         raise serializers.ValidationError('Invalid category_id')
     self.data['category'] = category
     return data

But it leads to an error.

When a serializer is passed a `data` keyword argument you must call `.is_valid()` before attempting to access the serialized `.data` representation.
You should either call `.is_valid()` first, or access `.initial_data` instead.

Is there a way to do this?

views.py

class SearchView(generics.GenericAPIView):
    def get(self, request):
        serializer = SearchSerializer(data=request.query_params)
        if serializer.is_valid():
            category_id = serializer.data['category_id']
            category = Category.objects.get(pk=category_id) ## Avoid this check
            # TODO: application logic
            return Response({'success': True})
        return Response(serializer.errors)

Answer 1

Instead of self.data['category'] = category you need to simply use data['category'] = category in your validate method

You should consider using a PrimaryKeyRelatedField though, like so:

category = serializers.PrimaryKeyRelatedField(
    queryset=Category.objects.all(),
    error_messages={'does_not_exist': 'Invalid category id'})

and you wouldn't need the validate at all and you would be able to access the entire Category object in your view directly as serializer.validated_data['category']