Filter the form's column choices

        This is done at the formset level as there's no other way i could find
        to get the parent object (stored in self.instance), and the form at the
        same time.

def add_fields(self, form, index):
        """Filter the form's column choices

        This is done at the formset level as there's no other way i could find
        to get the parent object (stored in self.instance), and the form at the
        same time.

        """
        super(ColumnFormSet, self).add_fields(form, index)
        form.fields['column'].choices = self.get_choices()