Django CreateView maintaining data between requests

Question

I'm currently using class-based views for a service that uses external APIs and facing a problem with CreateView that does not maintain initial data between GET (rendering the form) and POST (uploading data).

This view/route actually gets data from a webservice and the user is just required to be shown the data and upload a file. The webservice request is wrapped in a fetch method of the model manager:

class OrderManager(models.Manager):
    def fetch(self, user):
        data = requests.get(...).json()

        return dict(student=user,
                    foo=data['bar'])

And for the CreateView:

class OrderForm(ModelForm):
    class Meta:
        fields = ['picture']
        model = Order

class OrderCreateView(LoginRequiredMixin, CreateView):
    form_class = OrderForm
    model = Order

    def get_initial(self):
        return self.model.objects.fetch(self.request.user)

It works fine, when the user hits GET /order/new/ it correctly renders the data fetched from the webservice and presents the file upload form. But when the form is submitted, it again runs get_initial(...) instead of maintaining data from the previous request, and hits the webservice once again.

I'd like to maintain this data between GET and POST requests so I don't need to fetch data again, how is it possible with class-based views, and preferably without a lot of extra work? I was thinking about caching the requests but it doesn't really solve the problem if the user just takes too long to submit.

Answer 1

You shouldn't expect the state to be stored on the Class-Based View between requests. From the Documentation,

Each request served by a class-based view has an independent state; therefore, it is safe to store state variables on the instance (i.e., self.foo = 3 is a thread-safe operation).

This is easy to misunderstand: Once the form has been rendered, your context is gone and on the next (POST) request your OrderCreateView will be instantiated.

It's unclear what your reason is for fetching the data on your model manager? How are you saving this data?

In any case, since it looks like you're dealing with JSON data and the file-size isn't large enough to warrant file-storage, I recommend you just use a Cache-Based Session.

Example

After making sure that SessionMiddleware is enabled, add this to your get_initial method.

class OrderCreateView(LoginRequiredMixin, CreateView):
    form_class = OrderForm
    model = Order

    def get_initial(self):
        # Inspect the user's session
        user_data = self.request.session.get('user_data', None)
        if user_data is None:
            user_data = self.model.objects.fetch(self.request.user)
            self.request.session['user_data'] = user_data
        return user_data

---

I'm still not sure where you're saving your data, but if it's in the form, you may want to access the session data in get_form_kwargs().