How to query for a referenced object property in Mongoose?

In Mongoose, if your models have reference relationships, you can use the populate() method to query referenced documents. This method allows you to automatically populate documents from other collections in the query results.

Example Scenario

Assume we have two models: one is User, and the other is Post. In the Post model, we store a reference to the user who posted (i.e., the User's ID).

javascript
const mongoose = require('mongoose');
const { Schema } = mongoose;

const userSchema = new Schema({
  name: String,
  email: String
});

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

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

Querying Referenced User Information

Now, if you want to retrieve a post and its author's detailed information, you can use the populate() method when querying Post to populate the author field.

javascript
Post.findById(postId)
  .populate('author')
  .exec((err, post) => {
    if (err) throw err;
    console.log(post.title); // Display the post title
    console.log(post.author.name); // Display the author's name
    console.log(post.author.email); // Display the author's email
  });

Selective Population of Fields

If you are only interested in specific fields of the referenced user, you can use the select option within the populate() method to limit the returned fields.

javascript
Post.findById(postId)
  .populate({
    path: 'author',
    select: 'name'
  })
  .exec((err, post) => {
    if (err) throw err;
    console.log(post.title); // Display the post title
    console.log(post.author.name); // Only display the author's name
    // Note: Since we did not select email, the following property will be undefined
    console.log(post.author.email); // undefined
  });

Summary

Using Mongoose's populate() method effectively queries and manages related data in MongoDB. This makes it easier to access and display data when dealing with complex data structures.