Model Relations in Mongoose - https://abskmj.github.io/notes/posts/topics/mongoose/model-relations/

index.js

const mongoose = require('mongoose')
const { MongoMemoryServer } = require('mongodb-memory-server')

const User = require('./user.model')
const Post = require('./post.model')

const start = async () => {
  // connect to a mongo database
  const mongodb = new MongoMemoryServer()
  const connectionUri = await mongodb.getUri()
  await mongoose.connect(
    connectionUri, { useNewUrlParser: true, useUnifiedTopology: true }
  )

  // add seed data
  const author = await User.create({
    name: 'abskmj',
    email: 'abskmj@email.com'
  })

  await Post.create([{
    name: 'Relation: Belongs To',
    author: author
  }, {
    name: 'Relation: Has One',
    author: author
  }])

  //= ==== RELATION: Belongs To =====//

  // posts without author
  const posts = await Post.find({})

  console.log(posts)

  /*
    {
      _id: 5f71af94e66f4728f68ffc06,
      name: 'Relation: Belongs To',
      author: 5f71af94e66f4728f68ffc05
    }
  */

  // posts with author details pre populated
  const postsWithAuthor = await Post.find({}).populate('author')

  console.log(postsWithAuthor)

  /*
    {
      _id: 5f71af94e66f4728f68ffc06,
      name: 'Relation: Belongs To',
      author: {
        _id: 5f71af94e66f4728f68ffc05,
        name: 'abskmj',
        email: 'abskmj@email.com'
      }
    }
  */

  //= ==== RELATION: Has Many / Has One =====//

  // user without posts
  const user = await User.findOne()

  console.log(user)

  /*
    {
      _id: 5f71ce43687f2a2fb97fa462,
      name: 'abskmj',
      email: 'abskmj@email.com',
      id: '5f71ce43687f2a2fb97fa462'
    }
  */

  // user with posts prepopulated
  const userWithPosts = await User.findOne().populate('posts')

  console.log(userWithPosts)

  /*
    {
      _id: 5f71ce43687f2a2fb97fa462,
      name: 'abskmj',
      email: 'abskmj@email.com',
      posts: [
        {
          _id: 5f71ce43687f2a2fb97fa463,
          name: 'Relation: Belongs To',
          author: 5f71ce43687f2a2fb97fa462
        },
        {
          _id: 5f71ce43687f2a2fb97fa464,
          name: 'Relation: Has One',
          author: 5f71ce43687f2a2fb97fa462
        }
      ],
      id: '5f71ce43687f2a2fb97fa462'
    }
  */

  // close mongo connection
  await mongoose.connection.close()
  await mongodb.stop()
}

start()

package.json

{
  "name": "mongoose-model-relations",
  "version": "1.0.0",
  "description": "Model Relations in Mongoose",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "lint": "standard",
    "start": "node .",
    "dev": "nodemon --exec \"standard --fix && node . \""
  },
  "author": "abskmj@gmail.com",
  "license": "MIT",
  "dependencies": {
    "mongodb-memory-server": "^6.8.0",
    "mongoose": "^5.9.26",
    "nodemon": "^2.0.4"
  },
  "devDependencies": {
    "standard": "^14.3.4"
  }
}

post.model.js

const mongoose = require('mongoose')

const { Schema } = mongoose

const schema = new Schema({
  name: {
    type: String,
    required: true
  },
  author: {
    type: Schema.Types.ObjectId,
    ref: 'User',
    required: true
  }
}, { versionKey: false })

module.exports = mongoose.model('Post', schema)

user.model.js

const mongoose = require('mongoose')

const { Schema } = mongoose

const schema = new Schema({
  name: {
    type: String,
    required: true
  },
  email: {
    type: String,
    required: true
  }
}, {
  toObject: { virtuals: true },
  toJSON: { virtuals: true },
  versionKey: false
})

schema.virtual('posts', {
  ref: 'Post',
  localField: '_id',
  foreignField: 'author'
})

module.exports = mongoose.model('User', schema)