クラスベースのビューでミックスインを使用する

注意

これは発展的なトピックです。これらのテクニックについて詳しく読む前に、Django's class-based views で動作のしくみを知っておくことをおすすめします。

Django のビルトインのクラスベースビューではたくさんの機能が準備されていますが、個別に使いたい機能もあるかもしれません。例えば、HTTP レスポンスを生成するテンプレートをレンダリングするビューを記述したいとき、TemplateView は使えない状況もあります; POST ではテンプレートをレンダリングするだけで、GET のときはまったく異なる処理がしたいときなどです。この場合、TemplateResponse を直接使えますが、コードが重複する結果となってしまいます。

この理由から、Django は個別の機能を提供する多くの mixin を用意しています。例えば、テンプレートのレンダリングは TemplateResponseMixin でカプセル化されています。Django のリファレンスドキュメントは full documentation of all the mixins を含んでいます。

コンテキストとテンプレートのレスポンス

2 つの中心的な mixin が用意されており、クラスベースビュー内のテンプレートを扱うインターフェースに一貫性を保ちやすくなっています。

TemplateResponseMixin

TemplateResponse を返すビルトインビューは全て、 TemplateResponseMixin が提供する render_to_response() メソッドを呼び出します。ほとんどの場合、これはあなたによって呼び出されます(たとえば、 TemplateView と DetailView)の両方で実装されている get() メソッドによって)。同様に、これをオーバーライドする必要はほとんどありませんが、Django テンプレートでレンダリングされていないものを返したい場合は、オーバーライドする必要があるでしょう。この例については、 JSONResponseMixin example の例を参照してください。

render_to_response() itself calls get_template_names(), which by default will 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 returns its keyword arguments, but it is common to override this to add more members to the dictionary. You can also use the extra_context attribute.

Django の一般的なクラスベースのビューを構築する

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: Django の1つのオブジェクトを対象とするビュー

オブジェクトの詳細を表示するためには、基本的に2つの作業が必要です。まずはオブジェクトを検索し、それから適切なテンプレートと、そのオブジェクトをコンテキストとして、 TemplateResponse を作成します。

オブジェクトを取得するために、 DetailView は SingleObjectMixin に依存しています。このクラスではリクエストURLに基づいてオブジェクトを見つけ出すメソッド get_object() を提供しています(このメソッドでは URLConfで宣言されている pk および slug キーワード引数を検索し、ビューの model 属性、あるいは提供されている場合は queryset 属性からオブジェクトを検索します)。 SingleObjectMixin はまた get_context_data() メソッドも提供しています。このメソッドでは、テンプレートレンダーに渡すコンテキストデータを提供するために、Django組み込みのクラスベースビュー全てで使われています。

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: Django の複数のオブジェクトを対象とするビュー

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 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.)

Django のクラスベースのビューのミックスインを使用する

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. 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.

警告

Not all mixins can be used together, and not all generic class based views can be used with all other mixins. Here we present a few examples that do work; if you want to bring together other functionality then you'll have to consider interactions between attributes and methods that overlap between the different classes you're using, and how method resolution order will affect which versions of the methods will be called in what order.

The reference documentation for Django's class-based views and class-based view mixins will help you in understanding which attributes and methods are likely to cause conflict between different classes and mixins.

If in doubt, it's often better to back off and base your work on View or TemplateView, perhaps with SingleObjectMixin and MultipleObjectMixin. Although you will probably end up writing more code, it is more likely to be clearly understandable to someone else coming to it later, and with fewer interactions to worry about you will save yourself some thinking. (Of course, you can always dip into Django's implementation of the generic class-based views for inspiration on how to tackle problems.)

ビューで SingleObjectMixin を使用する

If we want to write a 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.

views.py
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 RecordInterestView(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 does with a 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:

urls.py
from django.urls import path
from books.views import RecordInterestView

urlpatterns = [
    #...
    path('author/<int:pk>/interest/', RecordInterestView.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.

ListView で SingleObjectMixin を使用する

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:

ListView` で使用するための Book の queryset
Since we have access to the Publisher whose books we want to list, we 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.

注釈

We have to think carefully about get_context_data(). Since both SingleObjectMixin and ListView will put things in the context data under the value of context_object_name if it's set, we'll instead explicitly ensure the Publisher is in the context data. ListView will add in the suitable page_obj and paginator for us providing we remember to call super().

Now we can write a new PublisherDetailView:

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

class PublisherDetailView(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().get(request, *args, **kwargs)

    def get_context_data(self, **kwargs):
        context = super().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:

ヒント

Each of your views should use only mixins or views from one of the groups of generic class-based views: detail, list, editing and date. For example it's fine to combine TemplateView (built in view) with MultipleObjectMixin (generic list), but you're likely to have problems combining SingleObjectMixin (generic detail) with MultipleObjectMixin (generic list).

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.

DetailView で FormMixin を使用する

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.

注釈

We use FormMixin and implement post() ourselves rather than try to mix DetailView with FormView (which provides a suitable post() already) because both of the views implement get(), and things would get much more confusing.

Our new AuthorDetailView 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 AuthorDetailView(FormMixin, DetailView):
    model = Author
    form_class = AuthorInterestForm

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

    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().form_valid(form)

get_success_url() provides somewhere to redirect to, which gets used in the default implementation of form_valid(). We have to provide our own post() as noted earlier.

より良い解決策

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, you could 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 less work than the above approach to have a separate view for processing the form, which could use FormView distinct from DetailView without concerns.

もう一つのより良い解決策

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 AuthorDetailView view is almost the same as when we first introduced AuthorDetailView; 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 import forms
from django.views.generic import DetailView
from books.models import Author

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

class AuthorDetailView(DetailView):
    model = Author

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

Then the AuthorInterestForm is a 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 AuthorDetailView is using on GET:

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

class AuthorInterestFormView(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().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 AuthorView 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 pass through keyword arguments to as_view() in the same way you would in your URLconf, such as if you wanted the AuthorInterestFormView behavior to also appear at another URL but using a different template:

from django.views import View

class AuthorView(View):

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

    def post(self, request, *args, **kwargs):
        view = AuthorInterestFormView.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.

単純な HTML を超える

クラスベースビューが役に立つのは、同じことを何度もしたい場合です。例えばAPIを作成している場合、全てのビューはレンダリングされたHTMLではなくJSONを返す必要があります。

全てのビューで利用するために、 JSONへの変換処理をするmixinクラスを作成することができます。

たとえば、JSONのmixinは以下のようになるでしょう。

from django.http import JsonResponse

class JSONResponseMixin:
    """
    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

注釈

Djangoモデルやクエリセットを正しくJSONに変換する方法についての詳細は Django オブジェクトのシリアル化 ドキュメントを参照してください。

このmixinは render_to_response() と同じ引数をもつ render_to_json_response() メソッドを提供します。これを使用するには、たとえば TemplateVIew へ取り入れ、 render_to_response() をオーバーライドして代わりに render_to_json_response() を呼び出す必要があります。

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 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)

このビューはレスポンスの作成を除いて、他の DetailView と同じように記述され、同じように動作します。

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 an HTTP header. 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().render_to_response(context)

Pythonのメソッド多重定義に関する仕様により、 super().render_to_response(context) の呼び出しは TemplateResponseMixin に実装された render_to_response() メソッドを呼び出します。

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