mongodb数据关联

const mongoose = require('mongoose');mongoose.connect('mongodb://localhost: 27017/mongo-relation', { useNewUrlParser: true, useUnifiedTopology: true }, err => {    if (err) {        console.log('数据库连接失败');    }    console.log('数据库连接成功');})

const categorySchema = new mongoose.Schema({    name: {        type: String    }}, {    toJSON: { virtuals: true }});categorySchema.virtual('posts', {    // 本地键    localField: '_id',    // 关联的模型    ref: 'Post',    // 外键    foreignField: 'categories',    // 输出多个    justOne: false});const Category = mongoose.model('Category', categorySchema);const postSchema = new mongoose.Schema({    title: {        type: String    },    body: {        type: String    },    category: {        type: mongoose.SchemaTypes.ObjectId,        ref: 'Category'    },    categories: [{        type: mongoose.SchemaTypes.ObjectId,        ref: 'Category'    }]});const Post = mongoose.model('Post', postSchema);async function init() {    // const cats = await Category.find().populate('categories');    // console.log(cats);    //     [    //   { _id: 5ee0a7f22adf1624e8b9b64d, name: 'nodejs', __v: 0 },    //   { _id: 5ee0a7f22adf1624e8b9b64e, name: 'vue.js', __v: 0 }    // ]    // 加上lean()     // const cats = await Category.find().populate('posts').lean();    // console.log(cats);    // [    //     {    //       _id: 5ee0a7f22adf1624e8b9b64d,    //       name: 'nodejs',    //       __v: 0,    //       posts: [ [Object] ]    //     },    //     {    //       _id: 5ee0a7f22adf1624e8b9b64e,    //       name: 'vue.js',    //       __v: 0,    //       posts: [ [Object], [Object] ]    //     }    //   ]    // cats[0].posts 查找第1个分类内容    // const cats = await Category.find().populate('posts').lean();    // console.log(cats[0].posts);    // [    //     {    //       _id: 5ee0a74dc6a921238454fa52,    //       title: '第2篇文章',    //       body: '内容2',    //       __v: 9,    //       category: 5ee0a7f22adf1624e8b9b64d,    //       categories: [ 5ee0a7f22adf1624e8b9b64d, 5ee0a7f22adf1624e8b9b64e ]    //     }    //   ]    // JSON.stringify    const cats = await Category.find().populate('posts').lean();    console.log(JSON.stringify(cats));    // [{"_id":"5ee0a7f22adf1624e8b9b64d","name":"nodejs","__v":0,    // "posts":[{"_id":"5ee0a74dc6a921238454fa52",    // "title":"第2篇文章","body":"内容2","__v":9,    // "category":"5ee0a7f22adf1624e8b9b64d",    // "categories":["5ee0a7f22adf1624e8b9b64d","5ee0a7f22adf1624e8b9b64e"]}]},    // {"_id":"5ee0a7f22adf1624e8b9b64e","name":"vue.js","__v":0,"posts":[{"_id":"5ee0a74dc6a921238454fa51","title":"第1篇文章","body":"内容1","__v":5,"category":"5ee0a7f22adf1624e8b9b64d","categories":["5ee0a7f22adf1624e8b9b64e"]},{"_id":"5ee0a74dc6a921238454fa52","title":"第2篇文章","body":"内容2","__v":9,"category":"5ee0a7f22adf1624e8b9b64d","categories":["5ee0a7f22adf1624e8b9b64d","5ee0a7f22adf1624e8b9b64e"]}]}]    // await Post.insertMany([{    //     title: '第1篇文章',    //     body: '内容1'    // }, {    //     title: '第2篇文章',    //     body: '内容2'    // }]);    // await Category.insertMany([{    //         name: 'nodejs'    //     },    //     {    //         name: 'vue.js'    //     }    // ]);    // const category = await Category.find();    // console.log(category);    // const cat1 = await Category.findOne({ name: 'nodejs' });    // const cat2 = await Category.findOne({ name: 'vue.js' });    // const post1 = await Post.findOne({ title: '第1篇文章' });    // const post2 = await Post.findOne({ title: '第2篇文章' });    // posts.category    // console.log(cat1);    // post1.categories = [cat2];    // post2.categories = [cat1, cat2];    // await post1.save();    // await post2.save();    // const post = await Post.find();    // console.log(post);    // console.log(post1, post2);    // const post = await Post.find().populate('categories')    // console.log(post[0], post[1]);}init()