Realtionship

By Reference(Normalization)

• Easier to maintain data consistency
• Only need to update 1 document if value changes

let author = {
  _id:"11223344",
  name:"James Peterson"
}
let course = {
  _id:"55667788",
  title:"Intro to Javascript",
  authors:["11223344"]
}

By Embedded(Denormalization)

• Faster performance
• Need to Update all related documents when value changes

let course = {
  _id:"55667788",
  title:"Intro to Javascript",
  authors:[{_id:"11223344",name:"James Peterson"}]
}

Hybrid

• Snapshot of data at a point in time
• e.g. an order has price of product at that time

let author = {
  _id:"11223344",
  name:"James Peterson",
  DOB:"1980-10-12",
  ...other properties
}
let course = {
  _id:"55667788",
  title:"Intro to Javascript",
  authors:[{_id:"11223344",name:"James Peterson"}]
}

Modeling by Reference

const authorSchema = new mongoose.Schema({
  name:String,
  bio:String,
  website:String
});
const Author = new mongoose.model('Author',authorSchema);
const courseSchema = new mongoose.Schema({
  title:String,
  author:{type:mongoose.Schema.Types.ObjectId,ref:'Author'}		//this references the author document by its _id
});
const Course = mongoose.model('Course',courseSchema);

Populate method

• reference documents in other collections

const listCourses = async () =>{
  const courses = await Course.find().populate('author')	//mongoose will get all the author details using the objectId
  res.send(result);
}

const listCourses = async () =>{
  const result = await Course.find().populate('author', 'name')	//mongoose will only get author name
  res.send(result);  
}

Modeling by Embedded

const authorSchema = new mongoose.Schema({
  name:String,
  bio:String,
  website:String
});
const Author = new mongoose.model('Author',authorSchema);
const courseSchema = new mongoose.Schema({
  title:String,
  author: authorSchema					//this embeds the author inside courseSchema
});
const Course = mongoose.model('Course',courseSchema);

add array of subdocument

const Course = mongoose.model("course", new mongoose.Schema({
  title:String,
  authors:[authorSchema]
}));
const createCourse = async (title, authors) =>{
	const course = new Course({title,authors});
	const result = await course.save();
	console.log(result);
}
createCourse("NodeJS Course", [new Author({name:"Peter"}),new Author({name:"James"})]);

const addAuthor = async (courseId,author){
  const course = await Course.findById(courseId);
  course.authors.push(author);
  course.save();
}
addAuthor("6234125asdf25c554werw87", new Author({name:"Janet"}));

remove subdocument

const removeAuthor = async (courseId,authorId){
  const course = await Course.findById(courseId);
  const author = course.authors.id(authorId);
  author.deleteOne();
  course.save();
}
removeAuthor("6234125asdf25c554","2348794j78678sd78");

remove subdocument directly in database

const removeAuthor = async (courseId){
  const course = await Course.update(_id:courseId,{
    $unset:{'author':''}
  });
}
removeAuthor("6234125asdf25c554");

update subdocument

const updateAuthor = async (courseId,newName){
  const course = await Course.findById(courseId);
  course.author.name = newName;
  course.save();
}
updateAuthor("6234125asdf25c554","James McGill");
update subdocument directly in database
const updateAuthor = async (courseId,newName){
  const course = await Course.update(_id:courseId,{
    $set:{'author.name':newName}
  });
}
updateAuthor("6234125asdf25c554","James McGill");