Mongoose allows documents to reference other documents. Referencing documents is known as normalization. It is good for consistency but creates more queries in the system.

Below, we have two models: Author & Course, and we are referencing an author document from the course schema.

const mongoose = require("mongoose");

const authorSchema = new mongoose.Schema({
   name: String,
   profession: String,
});

const Author = mongoose.model("Author", authorSchema);

module.exports = Author;

const mongoose = require("mongoose");

const Author = require("./authorModel");

const courseSchema = new mongoose.Schema({
    name: String,
    price: Number,
    author: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "Author",
    },
});

const Course = mongoose.model("Course", courseSchema);

module.exports = Course;

The value of the ref property is usually the name of the model mongoose should use to load referenced documents.

We create an author document.

const Author = require("./models/authorModel");
const Course = require("./models/courseModel");


const createAuthor = async (name, profession) => {
    const author = new Author({
       name,
       profession,
    });

    const result = await author.save();

    console.log(result)
};

createAuthor("Hemanta", "Full-stack Developer");

{
   _id: 60b606c4f2e9693af47f62c8,
   name: 'Hemanta',
   profession: 'Full-stack Developer',
   __v: 0
}

Now, we create a course document.

const createCourse = async (name, price, author) => {
  const course = new Course({
    name,
    price,
    author,
  })

  const result = await course.save()
  console.log(result)
}

createCourse("Master Node", 19.99, "60b606c4f2e9693af47f62c8")

{
   _id: 60b6075e9941a80730a431f1,
   name: 'Master Node',
   price: 19.99,
   author: 60b606c4f2e9693af47f62c8,
   __v: 0
}

Now we fetch the course we created.

const listCourses = async () => {
  const courses = await Course.find()
  console.log(courses)
}

listCourses()

{
  _id: 60b6075e9941a80730a431f1,
  name: 'Master Node',
  price: 19.99,
  author: 60b606c4f2e9693af47f62c8,
  __v: 0
}

We only see the author Id. If we want to see all the properties of the author document, we can use the populate method.

Population

Population is the process of automatically replacing the specified paths in the document with documents from other collections. We can load referenced documents using the populate() method.

const listCourses = async () => {
  const courses = await Course.find().populate("author")
  console.log(courses)
}

listCourses()

Including property

The author property of a course object can contain many properties. So, we can choose to include or exclude certain properties by passing a second argument to the populate method.

const listCourses = async () => {
  const courses = await Course.find().populate("author", "name")
  console.log(courses)
}

listCourses()

Excluding property

Similarly, we can choose to exclude properties of the referenced document using a dash(-). Below, we have included the name property and excluded the _id property of the author document.

const listCourses = async () => {
  const courses = await Course.find().populate("author", "name -_id")
  console.log(courses)
}

listCourses()