Association by MongoDB (2 ways: embedding data & object reference)

embed.js

var mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/blog_demo", { useNewUrlParser: true });
// POST - title, content
var postSchema = new mongoose.Schema({
    title: String,
    content: String
});

var Post = mongoose.model("Post", postSchema);

// USER - email, name
var userSchema = new mongoose.Schema({
    email: String,
    name: String,
    posts: [postSchema] // have to be the Schema name
});

var User = mongoose.model("User", userSchema);

// var newUser = new User({
//     email: "shan@gmail.com",
//     name : "shan"
// });

// newUser.posts.push({
//     title: "yeah",
//     content: "oh"
// });

// newUser.save(function(err, user){
//     if(err){
//         console.log(err);
//     } else {
//         console.log(user);
//     }
// });

User.findOne({email:"shan@gmail.com"}, function(err, user){
    if(err){
        console.log(err);
    } else {
        user.posts.push({
            title: "cool",
            content: "no"
        });
        user.save(function(err, user){
            if(err){
                console.log(err);
            } else {
                console.log(user);
            }
        })
    }
});

// var newPost = new Post({
//     title: "food",
//     content: "yami"
// });

// newPost.save(function(err, post){
//     if(err){
//         console.log(err);
//     } else {
//         console.log(post);
//     }
// })

reference.js

var mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/blog_demo_3", { useNewUrlParser: true });
// POST - title, content
var postSchema = new mongoose.Schema({
    title: String,
    content: String
});

var Post = mongoose.model("Post", postSchema);

// USER - email, name
var userSchema = new mongoose.Schema({
    email: String,
    name: String,
    posts: [
        {
            type: mongoose.Schema.Types.ObjectId,
            ref:"Post"
        }
    ] 
});

var User = mongoose.model("User", userSchema);

User.create({
    email: "shan@gmail.com",
    name: "shan"
});

// Create a new post for the user named "shan"
// Post.create({
//     title:"title_2",
//     content:"content_2"
// },function(err, post){
//     if(err){
//         console.log(err);
//     } else {
//       User.findOne({name:"shan"}, function(err, foundUser){
//             if(err){
//                 console.log(err);
//             } else {
//                 foundUser.posts.push(post);
//                 foundUser.save(function(err, data){
//                     if(err){
//                         console.log(err);
//                     } else {
//                         console.log(data);
//                     }
//                 });
//             }
//         });
//     }
// });

// Show all the posts
User.findOne({name:"shan"}).populate("posts").exec(function(err, user){
    if(err){
        console.log(err);
    } else {
        console.log(user);
    }
});