CreateView set foreign key from URL parameter

Question

I want to be able to create a lorum, using a ipsum_id (ForeignKey) that has been supplied in the URL. Using Django's class based views.

Given the following URL:

url(r'^lorem/create/ipsum/(?P<ipsum_id>\d+)/$', LoremCreateView.as_view(), name='create'),

I have the following view:

class LoremCreateView(CreateView):
    """ Allow a user to create a Lorem. """
    model = Lorem
    ...

    def get_context_data(self, **kwargs):
        """ Get context variables for rendering the template. """
        ipsum = get_object_or_404(Ipsum, pk=self.kwargs['ipsum_id'])
        kwargs['ipsum'] = ipsum
        return super().get_context_data(**kwargs)

    def form_valid(self, form):
        """ Save the form instance. """
        ipsum = get_object_or_404(Ipsum, pk=self.kwargs['ipsum_id'])
        form.instance.ipsum = ipsum
        return super().form_valid(form)

This works fine, however I don't like the fact I call get_object_or_404 twice.

Is there a better way of doing this?

It might be possible to override an appropriate method and set self.ipsum - not sure I like doing this either.

Also is get_object_or_404 appropriate here?

Matt · Accepted Answer · 2018-10-31 09:13:31Z

There's no need to add the ipsum instance to the template context unless it's used in the template. Adding things to self is the cleanest way.

Using get_object_or_404 is appropriate if we require the ipsum instance to exist in order to save a lorem.

You should end up with something like this:

# urls.py

from django.urls import path

urlpatterns = [
    path('lorem/create/ipsum/<int:ipsum_id>/', LoremCreateView.as_view(), name='create'),
]

# views.py

class LoremCreateView(CreateView):
    model = Lorem

    def dispatch(self, request, *args, **kwargs):
        """
        Overridden so we can make sure the `Ipsum` instance exists
        before going any further.
        """
        self.ipsum = get_object_or_404(Ipsum, pk=kwargs['ipsum_id'])
        return super().dispatch(request, *args, **kwargs)

    def form_valid(self, form):
        """
        Overridden to add the ipsum relation to the `Lorem` instance.
        """
        form.instance.ipsum = self.ipsum
        return super().form_valid(form)

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