mongodb數據關聯

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost: 27017/mongo-relation', { useNewUrlParser: true, useUnifiedTopology: true }, err => {
    if (err) {
        console.log('數據庫鏈接失敗');
    }
    console.log('數據庫鏈接成功');
})

const categorySchema = new mongoose.Schema({
    name: {
        type: String
    }

}, {
    toJSON: { virtuals: true }
});

categorySchema.virtual('posts', {
    // 本地鍵
    localField: '_id',
    // 關聯的模型
    ref: 'Post',
    // 外鍵
    foreignField: 'categories',
    // 輸出多個
    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個分類內容
    // const cats = await Category.find().populate('posts').lean();
    // console.log(cats[0].posts);
    // [
    //     {
    //       _id: 5ee0a74dc6a921238454fa52,
    //       title: '第2篇文章',
    //       body: '內容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":"內容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":"內容1","__v":5,"category":"5ee0a7f22adf1624e8b9b64d","categories":["5ee0a7f22adf1624e8b9b64e"]},{"_id":"5ee0a74dc6a921238454fa52","title":"第2篇文章","body":"內容2","__v":9,"category":"5ee0a7f22adf1624e8b9b64d","categories":["5ee0a7f22adf1624e8b9b64d","5ee0a7f22adf1624e8b9b64e"]}]}]

    // await Post.insertMany([{
    //     title: '第1篇文章',
    //     body: '內容1'
    // }, {
    //     title: '第2篇文章',
    //     body: '內容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()