Using mixins with class-based views

Django’s built-in class-based views provide a lot of functionality, but some of it you may want to use separately. For instance, you may want to write a view that renders a template to make the HTTP response, but you can’t use TemplateView; perhaps you need to render a template only on POST, with GET doing something else entirely. While you could use TemplateResponse directly, this will likely result in duplicate code.

For this reason, Django also provides a number of mixins that provide more discrete functionality. Template rendering, for instance, is encapsulated in the TemplateResponseMixin. The Django reference documentation contains full documentation of all the mixins.

Context and template responses

Two central mixins are provided that help in providing a consistent interface to working with templates in class-based views.

TemplateResponseMixin

Every built in view which returns a TemplateResponse will call the render_to_response() method that TemplateResponseMixin provides. Most of the time this will be called for you (for instance, it is called by the get() method implemented by both TemplateView and DetailView); similarly, it’s unlikely that you’ll need to override it, although if you want your response to return something not rendered via a Django template then you’ll want to do it. For an example of this, see the JSONResponseMixin example.

render_to_response() itself calls get_template_names(), which by default will just look up template_name on the class-based view; two other mixins (SingleObjectTemplateResponseMixin and MultipleObjectTemplateResponseMixin) override this to provide more flexible defaults when dealing with actual objects.

ContextMixin

Every built in view which needs context data, such as for rendering a template (including TemplateResponseMixin above), should call get_context_data() passing any data they want to ensure is in there as keyword arguments. get_context_data() returns a dictionary; in ContextMixin it simply returns its keyword arguments, but it is common to override this to add more members to the dictionary.

Building up Django’s generic class-based views

Let’s look at how two of Django’s generic class-based views are built out of mixins providing discrete functionality. We’ll consider DetailView, which renders a “detail” view of an object, and ListView, which will render a list of objects, typically from a queryset, and optionally paginate them. This will introduce us to four mixins which between them provide useful functionality when working with either a single Django object, or multiple objects.

There are also mixins involved in the generic edit views (FormView, and the model-specific views CreateView, UpdateView and DeleteView), and in the date-based generic views. These are covered in the mixin reference documentation.

DetailView: working with a single Django object

To show the detail of an object, we basically need to do two things: we need to look up the object and then we need to make a TemplateResponse with a suitable template, and that object as context.

To get the object, DetailView relies on SingleObjectMixin, which provides a get_object() method that figures out the object based on the URL of the request (it looks for pk and slug keyword arguments as declared in the URLConf, and looks the object up either from the model attribute on the view, or the queryset attribute if that’s provided). SingleObjectMixin also overrides get_context_data(), which is used across all Django’s built in class-based views to supply context data for template renders.

To then make a TemplateResponse, DetailView uses SingleObjectTemplateResponseMixin, which extends TemplateResponseMixin, overriding get_template_names() as discussed above. It actually provides a fairly sophisticated set of options, but the main one that most people are going to use is <app_label>/<model_name>_detail.html. The _detail part can be changed by setting template_name_suffix on a subclass to something else. (For instance, the generic edit views use _form for create and update views, and _confirm_delete for delete views.)

ListView: working with many Django objects

Lists of objects follow roughly the same pattern: we need a (possibly paginated) list of objects, typically a QuerySet, and then we need to make a TemplateResponse with a suitable template using that list of objects.

To get the objects, ListView uses MultipleObjectMixin, which provides both get_queryset() and paginate_queryset(). Unlike with SingleObjectMixin, there’s no need to key off parts of the URL to figure out the queryset to work with, so the default just uses the queryset or model attribute on the view class. A common reason to override get_queryset() here would be to dynamically vary the objects, such as depending on the current user or to exclude posts in the future for a blog.

MultipleObjectMixin also overrides get_context_data() to include appropriate context variables for pagination (providing dummies if pagination is disabled). It relies on object_list being passed in as a keyword argument, which ListView arranges for it.

To make a TemplateResponse, ListView then uses MultipleObjectTemplateResponseMixin; as with SingleObjectTemplateResponseMixin above, this overrides get_template_names() to provide a range of options, with the most commonly-used being <app_label>/<model_name>_list.html, with the _list part again being taken from the template_name_suffix attribute. (The date based generic views use suffixes such as _archive, _archive_year and so on to use different templates for the various specialized date-based list views.)

Using Django’s class-based view mixins

Now we’ve seen how Django’s generic class-based views use the provided mixins, let’s look at other ways we can combine them. Of course we’re still going to be combining them with either built-in class-based views, or other generic class-based views, but there are a range of rarer problems you can solve than are provided for by Django out of the box.

Using SingleObjectMixin with View

If we want to write a simple class-based view that responds only to POST, we’ll subclass View and write a post() method in the subclass. However if we want our processing to work on a particular object, identified from the URL, we’ll want the functionality provided by SingleObjectMixin.

We’ll demonstrate this with the Author model we used in the generic class-based views introduction.

from django.http import HttpResponseForbidden, HttpResponseRedirect
from django.urls import reverse
from django.views import View
from django.views.generic.detail import SingleObjectMixin
from books.models import Author

class RecordInterest(SingleObjectMixin, View):
    """Records the current user's interest in an author."""
    model = Author

    def post(self, request, *args, **kwargs):
        if not request.user.is_authenticated:
            return HttpResponseForbidden()

        # Look up the author we're interested in.
        self.object = self.get_object()
        # Actually record interest somehow here!

        return HttpResponseRedirect(reverse('author-detail', kwargs={'pk': self.object.pk}))

In practice you’d probably want to record the interest in a key-value store rather than in a relational database, so we’ve left that bit out. The only bit of the view that needs to worry about using SingleObjectMixin is where we want to look up the author we’re interested in, which it just does with a simple call to self.get_object(). Everything else is taken care of for us by the mixin.

We can hook this into our URLs easily enough:

from django.conf.urls import url
from books.views import RecordInterest

urlpatterns = [
    #...
    url(r'^author/(?P<pk>[0-9]+)/interest/$', RecordInterest.as_view(), name='author-interest'),
]

Note the pk named group, which get_object() uses to look up the Author instance. You could also use a slug, or any of the other features of SingleObjectMixin.

Using SingleObjectMixin with ListView

ListView provides built-in pagination, but you might want to paginate a list of objects that are all linked (by a foreign key) to another object. In our publishing example, you might want to paginate through all the books by a particular publisher.

One way to do this is to combine ListView with SingleObjectMixin, so that the queryset for the paginated list of books can hang off the publisher found as the single object. In order to do this, we need to have two different querysets:

Book queryset for use by ListView

Since we have access to the Publisher whose books we want to list, we simply override get_queryset() and use the Publisher’s reverse foreign key manager.

Publisher queryset for use in get_object()

We’ll rely on the default implementation of get_object() to fetch the correct Publisher object. However, we need to explicitly pass a queryset argument because otherwise the default implementation of get_object() would call get_queryset() which we have overridden to return Book objects instead of Publisher ones.

Now we can write a new PublisherDetail:

from django.views.generic import ListView
from django.views.generic.detail import SingleObjectMixin
from books.models import Publisher

class PublisherDetail(SingleObjectMixin, ListView):
    paginate_by = 2
    template_name = "books/publisher_detail.html"

    def get(self, request, *args, **kwargs):
        self.object = self.get_object(queryset=Publisher.objects.all())
        return super(PublisherDetail, self).get(request, *args, **kwargs)

    def get_context_data(self, **kwargs):
        context = super(PublisherDetail, self).get_context_data(**kwargs)
        context['publisher'] = self.object
        return context

    def get_queryset(self):
        return self.object.book_set.all()

Notice how we set self.object within get() so we can use it again later in get_context_data() and get_queryset(). If you don’t set template_name, the template will default to the normal ListView choice, which in this case would be "books/book_list.html" because it’s a list of books; ListView knows nothing about SingleObjectMixin, so it doesn’t have any clue this view is anything to do with a Publisher.

The paginate_by is deliberately small in the example so you don’t have to create lots of books to see the pagination working! Here’s the template you’d want to use:

{% extends "base.html" %}

{% block content %}
    <h2>Publisher {{ publisher.name }}</h2>

    <ol>
      {% for book in page_obj %}
        <li>{{ book.title }}</li>
      {% endfor %}
    </ol>

    <div class="pagination">
        <span class="step-links">
            {% if page_obj.has_previous %}
                <a href="?page={{ page_obj.previous_page_number }}">previous</a>
            {% endif %}

            <span class="current">
                Page {{ page_obj.number }} of {{ paginator.num_pages }}.
            </span>

            {% if page_obj.has_next %}
                <a href="?page={{ page_obj.next_page_number }}">next</a>
            {% endif %}
        </span>
    </div>
{% endblock %}

Avoid anything more complex

Generally you can use TemplateResponseMixin and SingleObjectMixin when you need their functionality. As shown above, with a bit of care you can even combine SingleObjectMixin with ListView. However things get increasingly complex as you try to do so, and a good rule of thumb is:

To show what happens when you try to get more sophisticated, we show an example that sacrifices readability and maintainability when there is a simpler solution. First, let’s look at a naive attempt to combine DetailView with FormMixin to enable us to POST a Django Form to the same URL as we’re displaying an object using DetailView.

Using FormMixin with DetailView

Think back to our earlier example of using View and SingleObjectMixin together. We were recording a user’s interest in a particular author; say now that we want to let them leave a message saying why they like them. Again, let’s assume we’re not going to store this in a relational database but instead in something more esoteric that we won’t worry about here.

At this point it’s natural to reach for a Form to encapsulate the information sent from the user’s browser to Django. Say also that we’re heavily invested in REST, so we want to use the same URL for displaying the author as for capturing the message from the user. Let’s rewrite our AuthorDetailView to do that.

We’ll keep the GET handling from DetailView, although we’ll have to add a Form into the context data so we can render it in the template. We’ll also want to pull in form processing from FormMixin, and write a bit of code so that on POST the form gets called appropriately.

Our new AuthorDetail looks like this:

# CAUTION: you almost certainly do not want to do this.
# It is provided as part of a discussion of problems you can
# run into when combining different generic class-based view
# functionality that is not designed to be used together.

from django import forms
from django.http import HttpResponseForbidden
from django.urls import reverse
from django.views.generic import DetailView
from django.views.generic.edit import FormMixin
from books.models import Author

class AuthorInterestForm(forms.Form):
    message = forms.CharField()

class AuthorDetail(FormMixin, DetailView):
    model = Author
    form_class = AuthorInterestForm

    def get_success_url(self):
        return reverse('author-detail', kwargs={'pk': self.object.pk})

    def get_context_data(self, **kwargs):
        context = super(AuthorDetail, self).get_context_data(**kwargs)
        context['form'] = self.get_form()
        return context

    def post(self, request, *args, **kwargs):
        if not request.user.is_authenticated:
            return HttpResponseForbidden()
        self.object = self.get_object()
        form = self.get_form()
        if form.is_valid():
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

    def form_valid(self, form):
        # Here, we would record the user's interest using the message
        # passed in form.cleaned_data['message']
        return super(AuthorDetail, self).form_valid(form)

get_success_url() is just providing somewhere to redirect to, which gets used in the default implementation of form_valid(). We have to provide our own post() as noted earlier, and override get_context_data() to make the Form available in the context data.

A better solution

It should be obvious that the number of subtle interactions between FormMixin and DetailView is already testing our ability to manage things. It’s unlikely you’d want to write this kind of class yourself.

In this case, it would be fairly easy to just write the post() method yourself, keeping DetailView as the only generic functionality, although writing Form handling code involves a lot of duplication.

Alternatively, it would still be easier than the above approach to have a separate view for processing the form, which could use FormView distinct from DetailView without concerns.

An alternative better solution

What we’re really trying to do here is to use two different class based views from the same URL. So why not do just that? We have a very clear division here: GET requests should get the DetailView (with the Form added to the context data), and POST requests should get the FormView. Let’s set up those views first.

The AuthorDisplay view is almost the same as when we first introduced AuthorDetail; we have to write our own get_context_data() to make the AuthorInterestForm available to the template. We’ll skip the get_object() override from before for clarity:

from django.views.generic import DetailView
from django import forms
from books.models import Author

class AuthorInterestForm(forms.Form):
    message = forms.CharField()

class AuthorDisplay(DetailView):
    model = Author

    def get_context_data(self, **kwargs):
        context = super(AuthorDisplay, self).get_context_data(**kwargs)
        context['form'] = AuthorInterestForm()
        return context

Then the AuthorInterest is a simple FormView, but we have to bring in SingleObjectMixin so we can find the author we’re talking about, and we have to remember to set template_name to ensure that form errors will render the same template as AuthorDisplay is using on GET:

from django.urls import reverse
from django.http import HttpResponseForbidden
from django.views.generic import FormView
from django.views.generic.detail import SingleObjectMixin

class AuthorInterest(SingleObjectMixin, FormView):
    template_name = 'books/author_detail.html'
    form_class = AuthorInterestForm
    model = Author

    def post(self, request, *args, **kwargs):
        if not request.user.is_authenticated:
            return HttpResponseForbidden()
        self.object = self.get_object()
        return super(AuthorInterest, self).post(request, *args, **kwargs)

    def get_success_url(self):
        return reverse('author-detail', kwargs={'pk': self.object.pk})

Finally we bring this together in a new AuthorDetail view. We already know that calling as_view() on a class-based view gives us something that behaves exactly like a function based view, so we can do that at the point we choose between the two subviews.

You can of course pass through keyword arguments to as_view() in the same way you would in your URLconf, such as if you wanted the AuthorInterest behavior to also appear at another URL but using a different template:

from django.views import View

class AuthorDetail(View):

    def get(self, request, *args, **kwargs):
        view = AuthorDisplay.as_view()
        return view(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        view = AuthorInterest.as_view()
        return view(request, *args, **kwargs)

This approach can also be used with any other generic class-based views or your own class-based views inheriting directly from View or TemplateView, as it keeps the different views as separate as possible.

More than just HTML

Where class-based views shine is when you want to do the same thing many times. Suppose you’re writing an API, and every view should return JSON instead of rendered HTML.

We can create a mixin class to use in all of our views, handling the conversion to JSON once.

For example, a simple JSON mixin might look something like this:

from django.http import JsonResponse

class JSONResponseMixin(object):
    """
    A mixin that can be used to render a JSON response.
    """
    def render_to_json_response(self, context, **response_kwargs):
        """
        Returns a JSON response, transforming 'context' to make the payload.
        """
        return JsonResponse(
            self.get_data(context),
            **response_kwargs
        )

    def get_data(self, context):
        """
        Returns an object that will be serialized as JSON by json.dumps().
        """
        # Note: This is *EXTREMELY* naive; in reality, you'll need
        # to do much more complex handling to ensure that arbitrary
        # objects -- such as Django model instances or querysets
        # -- can be serialized as JSON.
        return context

This mixin provides a render_to_json_response() method with the same signature as render_to_response(). To use it, we simply need to mix it into a TemplateView for example, and override render_to_response() to call render_to_json_response() instead:

from django.views.generic import TemplateView

class JSONView(JSONResponseMixin, TemplateView):
    def render_to_response(self, context, **response_kwargs):
        return self.render_to_json_response(context, **response_kwargs)

Equally we could use our mixin with one of the generic views. We can make our own version of DetailView by mixing JSONResponseMixin with the django.views.generic.detail.BaseDetailView – (the DetailView before template rendering behavior has been mixed in):

from django.views.generic.detail import BaseDetailView

class JSONDetailView(JSONResponseMixin, BaseDetailView):
    def render_to_response(self, context, **response_kwargs):
        return self.render_to_json_response(context, **response_kwargs)

This view can then be deployed in the same way as any other DetailView, with exactly the same behavior – except for the format of the response.

If you want to be really adventurous, you could even mix a DetailView subclass that is able to return both HTML and JSON content, depending on some property of the HTTP request, such as a query argument or a HTTP header. Just mix in both the JSONResponseMixin and a SingleObjectTemplateResponseMixin, and override the implementation of render_to_response() to defer to the appropriate rendering method depending on the type of response that the user requested:

from django.views.generic.detail import SingleObjectTemplateResponseMixin

class HybridDetailView(JSONResponseMixin, SingleObjectTemplateResponseMixin, BaseDetailView):
    def render_to_response(self, context):
        # Look for a 'format=json' GET argument
        if self.request.GET.get('format') == 'json':
            return self.render_to_json_response(context)
        else:
            return super(HybridDetailView, self).render_to_response(context)

Because of the way that Python resolves method overloading, the call to super(HybridDetailView, self).render_to_response(context) ends up calling the render_to_response() implementation of TemplateResponseMixin.