django.core.exceptions.ImproperlyConfigured: Could

Question

TL;DR: I am getting this error and don't know why:

django.core.exceptions.ImproperlyConfigured: Could not resolve URL for hyperlinked relationship using view name "user-detail". You may have failed to include the related model in your API, or incorrectly configured the 'lookup_field' attribute on this field.

I am going through the django-rest-framework tutorial and am currently at a point where function based views (FBV) were switched to class, mixin, and generic based views (CBV, MBV, GBV respectively). After switching to GBV, when I went to test my API, I received this error AssertionError: Expected view SnippetDetail to be called with a URL keyword argument named "pk". Fix your URL conf, or set the '.lookup_field' attribute on the view correctly.. I did some research and found that lookup_field needs to be set to the in the urlpatterns. Currently, my urls.py look like this:

from django.conf.urls import url, include
from rest_framework.urlpatterns import format_suffix_patterns
from snippets import views

# API endpoints
urlpatterns = format_suffix_patterns([
    url(r'^$', views.api_root),
    url(r'^snippets/$',
        views.SnippetList.as_view(),
        name='snippet-list'),
    url(r'^snippets/(?P<id>[0-9]+)/$',
        views.SnippetDetail.as_view(),
        name='snippet-detail'),
    url(r'^users/$',
        views.UserList.as_view(),
        name='user-list'),
    url(r'^users/(?P<id>[0-9]+)/$',
        views.UserDetail.as_view(),
        name='user-detail')
])

# Login and logout views for the browsable API
urlpatterns += [
    url(r'^auth/', include('rest_framework.urls',
                           namespace='rest_framework')),
]

and my views.py look like so:

from snippets.models import Snippet
from snippets.serializers import SnippetSerializer, UserSerializer
from snippets.permissions import IsOwnerOrReadOnly

from rest_framework import generics
from rest_framework import permissions
from rest_framework.decorators import api_view
from rest_framework.response import Response
from rest_framework.reverse import reverse

from django.contrib.auth.models import User


@api_view(['GET'])
def api_root(request, format=None):
    return Response({
        'users': reverse('user-list', request=request, format=format),
        'snippets': reverse('snippet-list', request=request, format=format)
    })


class UserList(generics.ListAPIView):
    queryset = User.objects.all()
    serializer_class = UserSerializer


class UserDetail(generics.RetrieveAPIView):
    queryset = User.objects.all()
    serializer_class = UserSerializer


class SnippetList(generics.ListCreateAPIView):
    queryset = Snippet.objects.all()
    serializer_class = SnippetSerializer
    permission_classes = (permissions.IsAuthenticatedOrReadOnly, IsOwnerOrReadOnly, )

    def perform_create(self, serializer):
        serializer.save(owner=self.request.user)


class SnippetDetail(generics.RetrieveUpdateDestroyAPIView):
    queryset = Snippet.objects.all()
    serializer_class = SnippetSerializer
    permission_classes = (permissions.IsAuthenticatedOrReadOnly, IsOwnerOrReadOnly, )

when I add lookup_field = 'id' in both UserDetail and SnippetDetail, the exception resolves itself. (Yay!). But, when I visit http://127.0.0.1/users/1/ ImproperlyConfigured: Could not resolve URL for hyperlinked relationship using view name "user-detail". You may have failed to include the related model in your API, or incorrectly configured the 'lookup_field' attribute on this field. is thrown. When I check the console, however, there is a second exception:

django.urls.exceptions.NoReverseMatch: Reverse for 'user-detail' with arguments '()' and keyword arguments '{'pk': 1}' not found. 2 pattern(s) tried: ['users/(?P[0-9]+)\.(?P[a-z0-9]+)/?$', 'users/(?P[0-9]+)/$']

During handling of the above exception, another exception occurred:

django.core.exceptions.ImproperlyConfigured: Could not resolve URL for hyperlinked relationship using view name "user-detail". You may have failed to include the related model in your API, or incorrectly configured the 'lookup_field' attribute on this field.

What I find interesting is that the kwargs for the first exception is {'pk': 1}, not {'id':1}. After some help from chat, someone pointed me to this piece of information:

Note that when using hyperlinked APIs you'll need to ensure that both the API views and the serializer classes set the lookup fields if you need to use a custom value.

This is useful as User serializer extends HyperlinkedModelSerializer:

from rest_framework import serializers

from django.contrib.auth.models import User

from snippets.models import Snippet

class UserSerializer(serializers.HyperlinkedModelSerializer):
    snippets = serializers.HyperlinkedRelatedField(many=True, view_name='snippet-detail', read_only=True)

    class Meta:
        model = User
        fields = ('url', 'id', 'username', 'snippets')

the User model and serializer has a reverse relationship with Snippet. Now, when I add lookup_field='id' to snippets attribute of UserSerializer (snippets = serializers.HyperlinkedRelatedField(many=True, view_name='snippet-detail', read_only=True, lookup_field='id')), like it asks me to do so here, the error is persistent.

What am I doing incorrectly? What can I do to fix this? Is it not having anything to do with lookup_id?

I understand that I could replace <id> with <pk> in my urlpatterns, but I would like to understand why this is happening.

Answer 1

I just had the same problem and I found, that the HyperlinkedIdentityField wants to insert some placeholder into your URL. But I had used URLs which did not require any placeholders to be set. A ListCreateAPIView to be precise:

url(
    r'^users/$',                  #<-- takes no params
    views.UserListView.as_view(), #<-- just prints a list 
    name="user-list"              #<-- HyperlinkedIdentityField pointing 
),                                #     here complained bitterly.

Answer 2

I eventually fixed my second exception by printing the serializer in the Django shell/python interactive console. The result I got was this:

>>> from snippets.serializers import UserSerializer
>>> print(UserSerializer())
UserSerializer():
    url = HyperlinkedIdentityField(view_name='user-detail')
    id = IntegerField(label='ID', read_only=True)
    username = CharField(help_text='Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.', max_length=150, validators=[<django.contrib.auth.validators.UnicodeUsernameValidator object>, <UniqueValidator(queryset=User.objects.all())>])
    snippets = HyperlinkedRelatedField(lookup_field='id', many=True, read_only=True, view_name='snippet-detail')

It turns out that to change <pk> to <id> in urlspatterns, you need to add url = HyperlinkedIdentityField(view_name='user-detail', lookup_field='id') in the UserSerializer class.