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Various programming stuff

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Django generic FormViews for objects

Introduction

We recently needed to create a number of views for changing the status of an Application model instance for our organization. An Application model instance can be filled and then cancelled, submitted, acceptted etc - for each of these status changes a form should be presented to the user. When the user submits the form the status of the Application will be changed.

To implement the above requirement we created a generic FormView that acts on the specific model instance. This used two basic CBV components: The FormView for the form manipulation and the SingleObjectMixing for the object handling.

Django Class Based Views (CBVs) can be used to create reusable Views using normal class inheritance. Most people use the well-known CreateView, UpdateView, DetailView and ListView, however, as we will see below, the FormView will help us write DRY code.

I have to notice here that an invaluable tool to help you understanding CBVs is the CBV inspector which has a nice web interface for browsing the CBV hierarchies, attributes and methods.

A quick introduction to the FormView

A simple FormView can be defined like this (CBV FormView):

class MyFormView(FormView):
  form_class = forms.MyFormView
  template_name = 'my_template.html'

The above can be used in urls.py like this:

urlpatterns = patterns('',
  url(r'^my_formview/$', views.MyFormView.as_view() , name='my_formview' ),

This will present a form to the user when he visits the my_formview url — however this form won’t do anything. To allow the form to actually do something when it’s been submitted we need to override the form_valid method.

def form_valid(self, form):
    value = form.cleaned_data['value']
    messages.info(self.request, "MyForm submitted with value {0}!".format(value) )
    return HttpResponseRedirect( reverse('my_formview') )

As you can see the submitted form is passed in the method and can be used to receive its cleaned_data. The FormView has various other options for instance a form_invalid method, an initial attribute to set the initial values for the form etc.

A quick introduction to the SingleObjectMixin

A SingleObjectMixin adds a number of attributes & methods to a view that can be used for object manipulation. The most important ones is the model and queryset attributes and the get_queryset and get_object(). To use the SingleObjectMixin in your CBV just add it to the list of the classes to inherit from and define either the model or the queryset attribute. After that you may pass a pk parameter to your view and you will get an object context variable in the template with the selected object!

Being generic and DRY

We can more or less now understand how we should use FormView and SingleObjectMixin to generate our generic FormView for acting on objects: Our FormView should get the object using the SingleObjectMixin and change it when the form is submitted using the values from the form. A first implementation would be the following:

class GenericObjectFormView1(FormView, SingleObjectMixin):

    def form_valid(self, form):
        obj = self.get_object()
        obj.change_status(form)
        return HttpResponseRedirect( obj.get_absolute_url() )

So our GenericObjectFormView1 class inherits from FormView and SingleObjectMixin. The only thing that we have to assure is that the Model we want to act on needs to implement a change_status method which gets the form and changes the status of that object based on its value. For instance, two implementations can be the following:

class CancelObjectFormView(GenericObjectFormView1):
    template_name = 'cancel.html'
    form_class = forms.CancelForm
    model = models.Application

class SubmitObjectFormView(GenericObjectFormView1):
    template_name = 'submit.html'
    form_class = forms.SubmitForm
    model = models.Application

Being more generic and DRY

The previous implementation has two problems:

  • What happens if the status of the object should not be changed even if the form is valid?
  • We shouldn’t need to create a new template for every new GenericObjectFormView since all these templates will just output the object information, ask a question for the status change and output the form.

Let’s write a new version of our GenericObjectFormView that actually resolves these:

class GenericObjectFormView2(FormView, SingleObjectMixin):
    template_name = 'generic_formview.html'
    ok_message = ''
    not_ok_message = ''
    title = ''
    question =''

    def form_valid(self, form):
        obj = self.get_object()
        r = obj.change_status(form)
        if r:
            messages.info(self.request, self.yes_message)
        else:
            messages.info(self.request, self.not_ok_message)
        return HttpResponseRedirect( obj.get_absolute_url() )

    def get_context_data(self, **kwargs):
        context = super(GenericYesNoFormView2, self).get_context_data(**kwargs)
        context['title'] = self.title
        context['question'] = self.question
        return context

The above adds an ok and not ok message which will be outputed if the status can or cannot be changed. To accomplish this, the change_status method should now return a boolean value to mark if the action was ok or not. Also, a generic template will now be used. This template has two placeholders: One for the title of the page (title attribute) and one for the question asked to the user (question attribute). Now we can use it like this:

class CancelObjectFormView(GenericObjectFormView2):
    form_class = forms.CancelForm
    model = models.Application
    ok_message = 'Cancel success!'
    not_ok_message = 'Not able to cancel!'
    title = 'Cancel an object'
    question = 'Do you want to cancel this object?'

class SubmitObjectFormView(GenericObjectFormView2):
    form_class = forms.SubmitForm
    model = models.Application
    ok_message = 'Submit  ok'
    not_ok_message = 'Cannot submit!'
    title = 'Submit an object'
    question ='Do you want to submit this object?'

Other options

We’ve just got a glimpse of how we can use CBVs to increase the DRYness of our Django applications. There are various extra things that we can add to our GenericObjectFormView2 as attributes which will be defined by inheriting classes. Some ideas is to check if the current user actually has access to modify the object (hint: override the get_object method of SingleObjectMixin) or render the form diffirently depending on the current user (hint: override the get_form_kwargs method of FormView).

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