Passing CBV data to a Form

I have just started working with Django, building an example application to find my way through the framework. Django seems to be more dynamic than I expected and the web is full of answers to questions I almost asked but help in no way at all.

I got totally stuck on this problem:

using a CBV and form_class, how can I limit a select in the form to a subset determined by a value known in the CBV

Essentially I wanted to pass a value from the View to the Form which  could then limit a selector. For example, suppose we have Clients who  have Contracts which have Tasks; when adding a Task for a Client we only  want to see Contracts for that specific Client.

Here's the basic structure:

#models.py

class Client(models.Model):
  name = models.CharField()

class Contract(models.Model):
  client = models.ForeignKey(Client)
  name = models.Charfield)

class Task(models.Model):
  contract = models.ForeignKey(Contract)
  notes = models.TextField()

From a detail page for a Client, we want to be able to add a task to a  Contract belong to the Client. We provide an URL to provide a form to  add the task

#urls.py
url(r'^clients/(?P<cid>\d+)/addtask$', view=TaskCreateView.as_view()),

This will pass a value cid to the class based view (CBV) representing the id of the Client. We  want the CBV to pass this value on to the form. Here's how we can do  this:

#views.py
class TaskCreateView(CreateView):
  model = Task
  template_name = 'task_form.html'
  form_class = TaskForm

  def get_form_kwargs(self, **kwargs):
    cid = self.kwargs.get('cid',0)
    kwargs = super(TaskCreateView, self).get_form_kwargs(**kwargs)
    kwargs['initial']['cid'] = cid
    return kwargs

  def get_context_data(self, **kwargs):
    context = super(TaskCreateView, self).get_context_data(**kwargs)
    if 'cid' in self.kwargs:
      context['client'] = get_object_or_404(Client, id=context['cid'])
    return context

Here the get_form_kwargs picks out cid and passes it to the form as a keyword argument. In get_context_data we also pick out cid this time fetching the Client record and returning it in the context so  the template can render appropriate information about the Client in the  Task form.

The Contract choices can now be specified in the form:

#forms.py
class TaskForm(forms.ModelForm):
  class Meta:
    model = Task

  def __init__(self, *args, **kwargs):
    super(TaskForm, self).__init__(*args, **kwargs)
    cid = kwargs['initial'].get('cid',0)
    if cid:
      self.fields['contract'].choices = ((c.id, c.name) for c in Contract.objects.filter

Here, as the form is being initialised, if a cid value has been passed from the View, we can filter the list of Contracts in the select widget