code: def add_fields(self, form, index):
          
          super(ColumnFormSet, self).add_fields(form, index)
          form.fields['column'].choices = self.get_choices()
docs: 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.