Tutorial 5 - Relationships & Hyperlinked APIs

At the moment relationships within our API are represented by using primary keys. In this part of the tutorial we'll improve the cohesion and discoverability of our API, by instead using hyperlinking for relationships.

Creating an endpoint for the root of our API

Right now we have endpoints for 'snippets' and 'users', but we don't have a single entry point to our API. To create one, we'll use a regular function-based view and the @api_view decorator we introduced earlier.

from rest_framework import renderers
from rest_framework.decorators import api_view
from rest_framework.response import Response
from rest_framework.reverse import reverse


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

Notice that we're using REST framework's reverse function in order to return fully-qualified URLs.

Creating an endpoint for the highlighted snippets

The other obvious thing that's still missing from our pastebin API is the code highlighting endpoints.

Unlike all our other API endpoints, we don't want to use JSON, but instead just present an HTML representation. There are two style of HTML renderer provided by REST framework, one for dealing with HTML rendered using templates, the other for dealing with pre-rendered HTML. The second renderer is the one we'd like to use for this endpoint.

The other thing we need to consider when creating the code highlight view is that there's no existing concreate generic view that we can use. We're not returning an object instance, but instead a property of an object instance.

Instead of using a concrete generic view, we'll use the base class for representing instances, and create our own .get() method.

class SnippetHighlight(generics.SingleObjectAPIView):
    model = Snippet
    renderer_classes = (renderers.StaticHTMLRenderer,)

    def get(self, request, *args, **kwargs):
        snippet = self.get_object()
        return Response(snippet.highlighted)

As usual we need to add the new views that we've created in to our URLconf. We'll add a url pattern for our new API root:

url(r'^$', 'api_root'),

And then add a url pattern for the snippet highlights:

url(r'^snippets/(?P<pk>[0-9]+)/highlight/$', views.SnippetHighlight.as_view()),

Hyperlinking our API

Dealing with relationships between entities is one of the more challenging aspects of Web API design. There are a number of different ways that we might choose to represent a relationship:

Using primary keys.
Using hyperlinking between entities.
Using a unique identifying slug field on the related entity.
Using the default string representation of the related entity.
Nesting the related entity inside the parent representation.
Some other custom representation.

REST framework supports all of these styles, and can apply them across forward or reverse relationships, or apply them across custom managers such as generic foreign keys.

In this case we'd like to use a hyperlinked style between entities. In order to do so, we'll modify our serializers to extend HyperlinkedModelSerializer instead of the existing ModelSerializer.

The HyperlinkedModelSerializer has the following differences from ModelSerializer:

It does not include the pk field by default.
It includes a url field, using HyperlinkedIdentityField.
Relationships use HyperlinkedRelatedField and ManyHyperlinkedRelatedField, instead of PrimaryKeyRelatedField and ManyPrimaryKeyRelatedField.

We can easily re-write our existing serializers to use hyperlinking.

class SnippetSerializer(serializers.HyperlinkedModelSerializer):
    owner = serializers.Field(source='owner.username')
    highlight = serializers.HyperlinkedIdentityField(view_name='snippet-highlight')

    class Meta:
        model = models.Snippet
        fields = ('url', 'highlight', 'owner',
                  'title', 'code', 'linenos', 'language', 'style')


class UserSerializer(serializers.HyperlinkedModelSerializer):
    snippets = serializers.ManyHyperlinkedRelatedField(view_name='snippet-detail')

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

Notice that we've also added a new 'highlight' field. This field is of the same type as the url field, except that it points to the 'snippet-highlight' url pattern, instead of the 'snippet-detail' url pattern.

Making sure our URL patterns are named

If we're going to have a hyperlinked API, we need to make sure we name our URL patterns. Let's take a look at which URL patterns we need to name.

The root of our API refers to 'user-list' and 'snippet-list'.
Our snippet serializer includes a field that refers to 'snippet-highlight'.
Our user serializer includes a field that refers to 'snippet-detail'.
Our snippet and user serializers include 'url' fields that by default will refer to '{model_name}-detail', which in this case will be 'snippet-detail' and 'user-detail'.

After adding all those names into our URLconf, our final 'urls.py' file should look something like this:

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

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

Reviewing our work

If we open a browser and navigate to the browseable API, you'll find that you can now work your way around the API simply by following links.

You'll also be able to see the 'highlight' links on the snippet instances, that will take you to the hightlighted code HTML representations.

We've now got a complete pastebin Web API, which is fully web browseable, and comes complete with authentication, per-object permissions, and multiple renderer formats.

We've walked through each step of the design process, and seen how if we need to customize anything we can gradually work our way down to simply using regular Django views.

You can review the final tutorial code on GitHub, or try out a live example in the sandbox.

Onwards and upwards

We've reached the end of our tutorial. If you want to get more involved in the REST framework project, here's a few places you can start:

Contribute on GitHub by reviewing and subitting issues, and making pull requests.
Join the REST framework discussion group, and help build the community.
Follow the author on Twitter and say hi.

Now go build some awesome things.

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