查找 Mongoose 集合中未被其他文档引用的文档

const mongoose = require('mongoose');

const schema = new mongoose.Schema({
    creator: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'User',
        validate: [mongoose.Types.ObjectId.isValid, 'Creator ID is invalid']
    },
    owner: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'User',
        validate: [mongoose.Types.ObjectId.isValid, 'Owner ID is invalid']
    },
    content: {
        type: String,
        required: 'Content is required'
    },
    likes: [
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: 'Like',
            validate: [mongoose.Types.ObjectId.isValid, 'Like ID is invalid']
        }
    ],
    replies: [
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: 'Post'
        }
    ],
    isOriginalPost: { // 新增字段
        type: Boolean,
        default: true // 默认为 true，表示原始帖子
    }
}, {
    autoCreate: true,
    timestamps: true
});

const Post = mongoose.model('Post', schema);

module.exports = Post;

async function createReplyPost(originalPostId, content, creator, owner) {
    const replyPost = new Post({
        creator: creator,
        owner: owner,
        content: content,
        isOriginalPost: false // 设置为 false
    });

    await replyPost.save();

    // 将回复帖子的 ID 添加到原始帖子的 replies 数组中
    const originalPost = await Post.findByIdAndUpdate(originalPostId, {
        $push: { replies: replyPost._id }
    });

    if (!originalPost) {
        throw new Error('Original post not found');
    }

    return replyPost;
}

async function getOriginalPosts() {
    const originalPosts = await Post.find({ isOriginalPost: true });
    return originalPosts;
}