Mongoose Virtual Populate Paginate

Introduction

Mongoose is a popular ODM library for MongoDB in Node.js. Virtual populate is a feature of Mongoose where we can attach virtual fields to our Mongoose schema that are not persisted in the database, but we can still query them as if they were.

Paginate is a module that simplifies the process of implementing server-side pagination in Node.js. The combination of virtual populate and pagination can be used to efficiently retrieve and display large data sets from MongoDB.

In this article, we will explore how to use Mongoose virtual populate with pagination in Node.js.

Prerequisites

Before we get started, make sure you have the following installed:

• Node.js
• MongoDB
• Mongoose
• Paginate

Virtual Populate

To use virtual populate in Mongoose, first, we need to define a virtual field in our schema. For example, let's say we have two models: User and Post. A user can have many posts, so we want to be able to retrieve a user with all their posts.

We can define a virtual field posts in our User schema that will populate all the posts for that user:

// user.js

const mongoose = require('mongoose');

const postSchema = new mongoose.Schema({
  title: { type: String, required: true },
  content: { type: String, required: true },
});

const userSchema = new mongoose.Schema({
  name: { type: String, required: true },
});

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

module.exports = { User: mongoose.model('User', userSchema), Post: mongoose.model('Post', postSchema) };

In the posts virtual, we define the following options:

• ref: The name of the model we want to populate. In our case, it's Post.
• localField: The name of the field in the current model that matches the foreign field. In our case, it's _id.
• foreignField: The name of the field in the referenced model that matches the local field. In our case, it's author.
• justOne: Whether to return a single document or an array of documents. In our case, we want to return an array, so it's false.

Now we can use this virtual posts field to retrieve a user with all their posts:

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

Pagination

To implement pagination in our Node.js application, we can use the paginate module. Install it using npm:

npm install paginate

The paginate module has a simple API that can be used to paginate any arbitrary query in Mongoose. For example, let's say we want to retrieve all the posts for a user with ID userId:

const query = Post.find({ author: userId });
const options = {
  page: 1,
  limit: 10,
};

const result = await paginate(query, options);

In the code above, we pass the Post query to paginate along with the pagination options. We set the current page to 1 and limit the number of results to 10.

The paginate method returns a promise that resolves with an object that contains the paginated results as well as some metadata about the pagination:

{
  data: [ /* paginated results */ ],
  meta: {
    total: 100,         // total number of documents
    limit: 10,          // limit per page
    page: 1,            // current page
    totalPages: 10,     // total number of pages
    hasNextPage: true,  // whether there is a next page
    hasPrevPage: false, // whether there is a previous page
  }
}

Virtual Populate with Pagination

Now that we know how to use virtual populate and pagination independently, let's combine them to efficiently retrieve and display large data sets from MongoDB.

We can retrieve a user with all their posts using virtual populate and then paginate the resulting array of posts using the paginate module. For example:

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

const query = Post.find({ _id: { $in: user.posts } });
const options = {
  page: 1,
  limit: 10,
};

const result = await paginate(query, options);

In the code above, we first retrieve the user with all their posts using virtual populate. We then pass the array of post IDs to the Post query and paginate the results.

Conclusion

We have learned how to use Mongoose virtual populate with pagination to efficiently retrieve and display large data sets from MongoDB in Node.js. By combining these two features, we can simplify the process of implementing server-side pagination and avoid performance issues with large data sets.