Bind an object (through a hidden field) to a form

gistfile1.py

class FormHiddenObjectMixin(object):
    """
    In forms in various create and update views, some of the form values
    are populated and hiddedn when the form is created. The user does not
    have to fill in this field. For example, when creating a shipment, we
    know that is for a particular purchase order. The purchase_order
    parameter of the form is set during form initialization.

    This mixin provides support for finding this related object either from
    URL query parameters (in the case of a CreateView) or from the object
    itself (in the case of an UpdateView).
    """
    hidden_object_model = None
    hidden_object_context_name = ''
    hidden_object_pk_query_param = ''
    hidden_object_field_name = ''
    hidden_object = None

    def get_hidden_object(self):
        if not self.hidden_object:
            self.hidden_object = self.get_hidden_object_from_object() or self.get_hidden_object_from_url()
        return self.hidden_object

    def get_hidden_object_from_object(self):
        if not self.object:
            return None
        if not self.hidden_object_field_name:
            raise ImproperlyConfigured(
                "FormHiddenObjectMixin requires the definition of 'hidden_object_field_name'")
        return getattr(self.object, self.hidden_object_field_name)

    def get_hidden_object_from_url(self):
        if not (self.hidden_object_pk_query_param and self.hidden_object_model):
            raise ImproperlyConfigured(
                "FormHiddenObjectMixin requires the definition of 'hidden_object_pk_query_param'"
                " and 'hidden_object_model'")
        pk = self.request.GET.get(self.hidden_object_pk_query_param, None)
        obj = get_object_or_404(self.hidden_object_model, pk=pk)
        return obj

    def get_hidden_initial_form_data(self):
        if not self.hidden_object_field_name:
            raise ImproperlyConfigured(
                "FormHiddenObjectMixin requires the definition of 'hidden_object_field_name'")
        return {
            self.hidden_object_field_name: self.get_hidden_object(),
        }

    def get_hidden_context_data(self):
        if not self.hidden_object_context_name:
            raise ImproperlyConfigured(
                "FormHiddenObjectMixin requires the definition of 'hidden_object_context_name'")
        return {
            self.hidden_object_context_name: self.get_hidden_object(),
        }

class PictureFormHiddenObjectMixin(FormHiddenObjectMixin):
    hidden_object_context_name = 'client'
    hidden_object_field_name = 'client'
    hidden_object_pk_query_param = 'client_pk'
    hidden_object_model = Client

class PictureCreate(PictureFormHiddenObjectMixin, CreateView):
    model = Picture
    form_class = modelform_factory(Picture,
        widgets={ 'client': forms.HiddenInput() },
        fields=('pic', 'client', ))

    def get_initial(self):
        initial = super(PictureCreate, self).get_initial()
        initial.update(self.get_hidden_initial_form_data())
        return initial

    def get_context_data(self, **kwargs):
        context = super(PictureCreate, self).get_context_data(**kwargs)
        context.update(self.get_hidden_context_data())
        return context

# My URL looks like example.com/upload?client_pk=3
# You could change the URL to example.com/client/3/upload/ as well. You'd have to slightly
# change the logic in get_hidden_object_from_url() to get it from self.kwargs
# rather than from the query parameter