猫鼬5.4.6鉴别器未显示

Question

在保存继承的对象时，不显示鉴别器，而且我似乎也找不到导致它的原因： 猫鼬：5.4.4， 节点：11.6.0 快递：4.16.4

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const options = {
    discriminatorKey: 'kind'
};

const AlternativeSchema = new Schema({

    createdAt: {
        type: Date,
        default: Date.now
    },
    questionId: {
        type: Schema.Types.ObjectId,
        ref: 'Question',
        required: true
    },
    belief: {
        type: Number,
        required: true,
        default: 0
    },
    baserate: {
        type: Number,
        required: true
    }
},
options);

module.exports = mongoose.model('Alternative', AlternativeSchema);

//子文件：

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const Alternative = require('./alternative.js');


const SingletonSchema = new Schema({
    title: {
        type: String,
        default: 'compound',
        required: true
    },
    desc: {
        type: String
    }
});

module.exports = Alternative.discriminator('Singleton', SingletonSchema);

当我执行以下操作时： const newSingleton = new Singleton（...） newSingleton.save（）;

不保存鉴别符。 有什么想法为什么会这样吗？