Class based views

Remarks#

When using CBV we often need to know exactly what methods we can overwrite for each generic class. This page of the django documentation lists all the generic classes with all of their methods flattened and the class attributes we can use.

In addition, Classy Class Based View website provides the same information with a nice interactive interface.

Class Based Views

Class based views let you focus on what make your views special.

A static about page might have nothing special, except the template used. Use a TemplateView! All you have to do is set a template name. Job done. Next.

views.py

from django.views.generic import TemplateView


class AboutView(TemplateView):
    template_name = "about.html"

urls.py

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

urlpatterns = [
    url('^about/', views.AboutView.as_view(), name='about'),
]

Notice how we don’t use directly AboutView in the url. That’s because a callable is expected and that’s exactly what as_view() return.

Context data

Sometimes, your template need a bit more of information. For example, we would like to have the user in the header of the page, with a link to their profile next to the logout link. In these cases, use the get_context_data method.

views.py

class BookView(DetailView):
    template_name = "book.html"

    def get_context_data(self, **kwargs)
        """ get_context_data let you fill the template context """
        context = super(BookView, self).get_context_data(**kwargs)
        # Get Related publishers
        context['publishers'] = self.object.publishers.filter(is_active=True)
        return context

You need to call get_context_data method on the super class and it will return the default context instance. Any item that you add to this dictionary will be available to the template.

book.html

<h3>Active publishers</h3>
<ul>
  {% for publisher in publishers %}
    <li>{{ publisher.name }}</li>
  {% endfor %}
</ul>

List and Details views

Template views are fine for static page and you could use them for everything with get_context_data but it would be barely better than using function as views.

Enter ListView and DetailView

app/models.py

from django.db import models

class Pokemon(models.Model):
    name = models.CharField(max_length=24)
    species = models.CharField(max_length=48)
    slug = models.CharField(max_length=48)

app/views.py

from django.views.generic import ListView, DetailView
from .models import Pokemon


class PokedexView(ListView):
    """ Provide a list of Pokemon objects """
    model = Pokemon
    paginate_by = 25

class PokemonView(DetailView):
    model = Pokemon

That’s all you need to generate a view listing all your objects of a models and views of singular item. The list is even paginated. You can provide template_name if you want something specific. By default, it’s generated from the model name.

app/templates/app/pokemon_list.html

<!DOCTYPE html>
<title>Pokedex</title>
<ul>{% for pokemon in pokemon_list %}
    <li><a href="{% url "app:pokemon" pokemon.pk %}">{{ pokemon.name }}</a>
        &ndash; {{ pokemon.species }}
</ul>

The context is populated with the list of object under two name, object_list and a second one build from the model name, here pokemon_list. If you have paginated the list, you have to take care of next and previous link too. The Paginator object can help with that, it’s available in the context data too.

app/templates/app/pokemon_detail.html

<!DOCTYPE html>
<title>Pokemon {{ pokemon.name }}</title>
<h1>{{ pokemon.name }}</h1>
<h2>{{ pokemon.species }} </h2>

As before, the context is populated with your model object under the name object and pokemon, the second one being derived from the model name.

app/urls.py

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

app_name = 'app'
urlpatterns = [
    url(r'^pokemon/$', views.PokedexView.as_view(), name='pokedex'),
    url(r'^pokemon/(?P<pk>\d+)/$', views.PokemonView.as_view(), name='pokemon'),
]

In this snippet, the url for the detail view is built using the primary key. It’s also possible to use a slug as argument. This gives a nicer looking url that’s easier to remember. However it requires the presence of a field named slug in your model.

url(r'^pokemon/(?P<slug>[A-Za-z0-9_-]+)/$', views.PokemonView.as_view(), name='pokemon'),

If a field called slug is not present, you can use the slug_field setting in DetailView to point to a different field.

For pagination, use a page get parameters or put a page directly in the url.

Form and object creation

Writing a view to create object can be quite boring. You have to display a form, you have to validate it, you have to save the item or return the form with an error. Unless you use one of the generic editing views.

app/views.py

from django.core.urlresolvers import reverse_lazy
from django.views.generic.edit import CreateView, UpdateView, DeleteView
from .models import Pokemon


class PokemonCreate(CreateView):
    model = Pokemon
    fields = ['name', 'species']


class PokemonUpdate(UpdateView):
    model = Pokemon
    fields = ['name', 'species']


class PokemonDelete(DeleteView):
    model = Pokemon
    success_url = reverse_lazy('pokedex')

CreateView and UpdateView have two required attribute, model and fields. By default, both use a template name based on the model name suffixed by ‘_form’. You can change only the suffix with the attribute template_name_suffix. The DeleteView show a confirmation message before deleting the object.

Both UpdateView and DeleteView need to fetch on object. They use the same method as DetailView, extracting variable from the url and matching the object fields.

app/templates/app/pokemon_form.html (extract)

<form action="" method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="Save" />
</form>

form contains the form with all needed fields. Here, it will be displayed with a paragraph for each field because of as_p.

app/templates/app/pokemon_confirm_delete.html (extract)

<form action="" method="post">
    {% csrf_token %}
    <p>Are you sure you want to delete "{{ object }}"?</p>
    <input type="submit" value="Confirm" />
</form>

The csrf_token tag is required because of django protection against request forgery. The attribute action is left empty as the url displaying the form is the same as the one handling the deletion/save.

Two issues remain with the model, if using the same as with the list and detail exemple. First, create and update will complain about a missing redirection url. That can be solved by adding a get_absolute_url to the pokemon model. Second issue is the deletion confirmation not displaying meaningful information. To solve this, the easiest solution is to add a string representation.

app/models.py

from django.db import models
from django.urls import reverse
from django.utils.encoding import python_2_unicode_compatible


@python_2_unicode_compatible
class Pokemon(models.Model):
    name = models.CharField(max_length=24)
    species = models.CharField(max_length=48)

    def get_absolute_url(self):
        return reverse('app:pokemon', kwargs={'pk':self.pk})

    def __str__(self):
        return self.name

The class decorator will make sure everything work smoothly under python 2.

Minimal example

Django Class Based Views: Example of CreateView

One View, Multiple Forms

Here is a quick example of using multiple forms in one Django view.

from django.contrib import messages
from django.views.generic import TemplateView

from .forms import AddPostForm, AddCommentForm
from .models import Comment

class AddCommentView(TemplateView):

    post_form_class = AddPostForm
    comment_form_class = AddCommentForm
    template_name = 'blog/post.html'

    def post(self, request):
        post_data = request.POST or None
        post_form = self.post_form_class(post_data, prefix='post')
        comment_form = self.comment_form_class(post_data, prefix='comment')

        context = self.get_context_data(post_form=post_form,
                                        comment_form=comment_form)

        if post_form.is_valid():
            self.form_save(post_form)
        if comment_form.is_valid():
            self.form_save(comment_form)

        return self.render_to_response(context)     

    def form_save(self, form):
        obj = form.save()
        messages.success(self.request, "{} saved successfully".format(obj))
        return obj

    def get(self, request, *args, **kwargs):
        return self.post(request, *args, **kwargs)