Mongoose & the Full Stack›Pro· 35 min read

Relationships & populate

Connect documents across collections — like posts to their authors — and pull them together with populate.

What you will learn

Reference one document from another
Use populate to join data
Decide between referencing and embedding

Referencing other documents

Real data is connected: a post has an author, an order has a customer. Store the related document’s _id as a reference.

A Post that references a User

const postSchema = new mongoose.Schema({
  title: String,
  body: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: "User" }
});

The author field does not store the whole user — it stores that user’s _id. The type mongoose.Schema.Types.ObjectId says “this holds an id”, and ref: "User" tells Mongoose which collection that id points to (the User model). It is like saving a friend’s phone number instead of writing out their entire contact card each time.

populate — pull in the related data

populate replaces the stored _id with the actual referenced document when you query.

populate joins the referenced data

// Without populate: author is just an id
// With populate: author becomes the full user document
const posts = await Post.find().populate("author");
// posts[0].author.name now works

Normally a post’s author comes back as just an id — not very useful on its own. Adding .populate("author") tells Mongoose to follow that id into the User collection and swap it for the whole user document. That is why posts[0].author.name suddenly works: author is now a full object, not a bare id.

Note: Output: Without populate → author: ObjectId('65f...u1') With populate → author: { _id: '65f...u1', name: 'Asha', email: 'asha@x.com' } Same query, but populate pulls the related user in so you can read its fields directly — just like a join in SQL.

Note: Reference (store an id) when data is shared or large; embed (nest the object) when data belongs only to its parent and is read together. Choosing well is a key data-modelling skill.

Q. What does Mongoose populate() do?

Answer: populate() follows a reference (stored _id) and replaces it with the full referenced document — like a join.

✍️ Practice

Create User and Post models where a post references a user.
Query posts with populate("author") and read the author’s name.

🏠 Homework

Model a blog with users, posts and comments using references, and populate them.