How does populate work in Mongoose? | MongoDB Interview Questions

What You'll Learn

Setting up references
Using populate
Deep population

Define References

code.jsJavaScript

const postSchema = new mongoose.Schema({
  title: String,
  content: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
  comments: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Comment' }]
});

const Post = mongoose.model('Post', postSchema);

Basic Populate

code.jsJavaScript

// Populate single field
const post = await Post.findById(id).populate('author');
// Result: { ..., author: { _id: ..., name: 'John', email: '...' } }

// Populate multiple fields
const post = await Post.findById(id)
  .populate('author')
  .populate('comments');

// Or
const post = await Post.findById(id).populate('author comments');

Select Fields

code.jsJavaScript

const post = await Post.findById(id)
  .populate('author', 'name email')  // Only name and email
  .populate('comments', '-__v');     // Exclude __v

Populate with Options

code.jsJavaScript

const post = await Post.findById(id).populate({
  path: 'comments',
  match: { approved: true },
  select: 'text createdAt',
  options: { sort: { createdAt: -1 }, limit: 5 }
});

Deep Populate

code.jsJavaScript

// Comment has author reference
const post = await Post.findById(id).populate({
  path: 'comments',
  populate: { path: 'author', select: 'name' }
});

Virtual Populate

code.jsJavaScript

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

const user = await User.findById(id).populate('posts');