[Fixed]-Django rest framework: Get detail view using a field other than primary key integer id

Add lookup_field to product_id:

class ProductDetailCustom(generics.RetrieveAPIView):
    lookup_field = "product_id"
    queryset = Product.objects.all()
    serializer_class = ProductCustomSerializer

The lookup_field would be used by the get_object call which would retrieve the model instance. So you can write a custom get_object method on your view.

Ref: http://www.django-rest-framework.org/api-guide/generic-views/#genericapiview