code: def shall_save(self, form, name, composite_form):
          
          if composite_form.empty_permitted and not composite_form.has_changed():
              return False
          return True
docs: Return ``True`` if the given ``composite_form`` (the nested form of
          this field) shall be saved. Return ``False`` if the form shall not be
          saved together with the super-form.
  
          By default it will return ``False`` if the form was not changed and the
          ``empty_permitted`` argument for the form was set to ``True``. That way
          you can allow empty forms.