Introduction to Mongoose Populate

Introduction to Mongoose Populate

Introduction to Mongoose Populate

In this post, we will give you information about Introduction to Mongoose Populate. Here we will give you detail about Introduction to Mongoose Populate And how to use it also give you a demo for it if it is necessary.

In Mongoose, populate lets you pullĀ  in referenced documents from another collection. Populate is similar to a left outer join in SQL, but the difference is that populate happens in your Node.js application rather than n the database server. Mongoose executes a separate query under the hood to load the referenced documents.

Basic Populate for Introduction to Mongoose Populate

Suppose you have two Mongoose models:
Movie and Person. Movie documents have a director and an array of actors.

const Person = mongoose.model('Person', mongoose.Schema({
  name: String

// 'ref' tells Mongoose populate what model to query
const Movie = mongoose.model('Movie', mongoose.Schema({
  title: String,
  director: {
    type: mongoose.ObjectId,
    ref: 'Person'
  actors: [{
    type: mongoose.ObjectId,
    ref: 'Person'

Mongoose queries have a populate() function that lets you load a movie and its corresponding director and actors in one line:

const people = await Person.create([
  { name: 'James Cameron' },
  { name: 'Arnold Schwarzenegger' },
  { name: 'Linda Hamilton' }
await Movie.create({
  title: 'Terminator 2',
  director: people[0]._id,
  actors: [people[1]._id, people[2]._id]

// Load just the movie's director
let movie = await Movie.findOne().populate('director');; // 'James Cameron'
movie.actors[0].name; // undefined

// Load both the director and the actors
movie = await Movie.findOne().populate('director').populate('actors');; // 'James Cameron'
movie.actors[0].name; // 'Arnold Schwarzenegger'
movie.actors[1].name; // 'Linda Hamilton'

Populate On Existing Documents

Mongoose documents also have a populate() function. Given an existing movie document, you
can populate() any number of paths. Just remember to call Document#execPopulate() to actually execute the populate() call.

// Load just the movie's director
let movie = await Movie.findOne();; // undefined
movie.actors[0].name; // undefined

// Populate the director
await movie.populate('director').execPopulate();; // 'James Cameron'
movie.actors[0].name; // undefined

// Populate the actors
await movie.populate('actors').execPopulate();; // 'James Cameron'
movie.actors[0].name; // 'Arnold Schwarzenegger'
movie.actors[1].name; // 'Linda Hamilton'

Edge Cases for Introduction to Mongoose Populate

If you’re populating a single document and the referenced document doesn’t exist,
Mongoose will set the populated property to null.

await Person.deleteOne({ name: 'James Cameron' });

const movie = await Movie.findOne().populate('director');
movie.director; // null

If you’re populating an array and one of the referenced documents doesn’t exist,
Mongoose will filter that value out of the array by default, returning a shorter
array. You can override this with the retainNullValues option.

await Person.deleteOne({ name: 'Arnold Schwarzenegger' });

let movie = await Movie.findOne().populate('actors');
movie.actors.length; // 1
movie.actors[0].name; // 'Linda Hamilton'

// Set 'retainNullValues' option to insert 'null' for
// missing documents in the array
movie = await Movie.findOne().populate({
  path: 'actors',
  options: { retainNullValues: true }
movie.actors.length; // 2
movie.actors[0]; // null
movie.actors[1].name; // 'Linda Hamilton'

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A mongoose is a small, carnivorous mammal that is found in Africa, Asia, and southern Europe. They are known for their ability to kill venomous snakes, and they have been used for centuries to control snake populations. Mongooses are also popular pets, but they can be difficult to care for and are not recommended for everyone.

Here are some of the pros and cons of owning a mongoose for Introduction to Mongoose Populate:


  • Mongooses are intelligent and can be trained to do tricks.
  • They are relatively small and easy to care for.
  • They can be effective at controlling snake populations.


  • Mongooses can be aggressive and unpredictable.
  • They are not legal to own in some places.
  • They can carry diseases, such as rabies.

If you are considering getting a mongoose as a pet, it is important to do your research and make sure that you are prepared to provide the proper care. Mongooses are not for everyone, but they can make great companions for the right people.

Here are some additional facts about mongooses:

  • They are about the size of a cat, with long bodies and tails.
  • They have sharp claws and teeth, which they use to catch and kill prey.
  • They are active during the day and night.
  • They eat a variety of foods, including snakes, rodents, insects, and fruit.
  • Mongooses live in a variety of habitats, including forests, grasslands, and deserts.
  • They are solitary animals, but they will sometimes gather in groups to hunt.
  • Mongooses are monogamous, and they mate for life.
  • Females give birth to litters of 2-4 young.
  • Mongooses reach sexual maturity at about 6 months old.
  • They have a lifespan of about 10 years.

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