def METHOD_NAME(self):
    # Ensure all locked questionnaires still have the same status (included or not)
    not_locked = []
    if self.cleaned_data.get("general_questionnaires"):
        not_locked = list(self.cleaned_data.get("general_questionnaires").filter(is_locked=False))
    locked = list(self.instance.general_contribution.questionnaires.filter(is_locked=True))
    if not not_locked + locked:
        self.add_error("general_questionnaires", _("At least one questionnaire must be selected."))
    return not_locked + locked