Mongoose: How to compare _id with a string value

Updated: December 30, 2023 By: Guest Contributor Post a comment


Working with MongoDB in a Node.js application, you’ll frequently encounter scenarios where you need to compare an Object ID (_id) with a string. Mongoose, a popular ODM (Object Data Modeling) library for MongoDB and Node.js, provides several ways to handle this operation. Understanding how to effectively compare these two seemingly different data types is essential for implementing functionality like searching, validation, and relational data querying. In this tutorial, we’ll explore various methods to compare Mongoose Object IDs with string values, providing code examples and explaining the intricacies involved in making these comparisons work seamlessly.


  • Basic understanding of Node.js and JavaScript/TypeScript
  • Familiarity with MongoDB and Mongoose ODM
  • Node.js environment setup along with NPM or Yarn for package management
  • A Mongoose model to work with

Understanding Object IDs

In MongoDB, every document stored in a collection has a unique identifier called ‘_id’. By default, MongoDB uses a 12-byte Object ID which is often represented as a 24-character hex string. Mongoose wraps these Object IDs with its own ObjectId type, enabling additional functionality. When comparing an Object ID with a string, it’s crucial to use methods that correctly handle this data type, ensuring a true comparison.

Comparing Object ID with String Directly

const mongoose = require('mongoose');

// Assuming we have a model 'User'
const User = mongoose.model('User', new mongoose.Schema());

// Function to find a user by _id as a string
const findUserByIdAsString = async (idString) => {
  try {
    const user = await User.findById(idString).exec();
  } catch (error) {
    console.error('Failed to find user:', error.message);

// Example usage

This code attempts to find a document based on a string input for the ‘_id’. Mongoose’s findById method implicitly converts the string to a Mongoose ObjectId and performs the comparison. If provided with an invalid format, it triggers an error.

Using equals Method for Comparison

const mongoose = require('mongoose');
const { ObjectId } = mongoose.Types;

// Comparison function that utilizes MongoDB ObjectId's 'equals' method
const doesIdMatch = (id1, id2) => {
  return new ObjectId(id1).equals(id2);

// Example usage
const idFromDb = new ObjectId();
const idString = idFromDb.toHexString();
console.assert(doesIdMatch(idFromDb, idString), 'Ids do match');

Here, we’re directly converting one ID to a ObjectId instance and comparing it using the built-in equals() method which can handle both ObjectId and its hexadecimal string representation.

Comparing for Equality in Query Syntax

When querying MongoDB using Mongoose, you can directly compare an Object ID to a string within a query. Mongoose will perform the necessary type conversion automatically.

const findUserByStringIdInQuery = async (idString) => {
  const users = await User.find({ _id: idString });
  // ... handle the found users

In this example, passing a string for the ‘_id’ field is implicitly converted and compared by the query process within Mongoose.

Advanced: Custom Schema Type

For a more seamless experience across your application, you can implement a custom Mongoose schema type that represents your ‘_id’ validity checks and directly allows string comparison in all parts of your code.

const CustomObjectId = require('./custom-object-id-type');

const MySchema = new mongoose.Schema({
  _id: { type: CustomObjectId, required: true }

// Then, you can simply use this schema in your model definitions

This advanced implementation defines your handling for ‘ObjectId’ that could encapsulate validation, casting, and comparison logic to better fit your application design.


In this extensive guide, we have covered multiple approaches to compare Mongoose Object IDs with string values. By understanding Mongoose’s automatic type conversion and the methods available for comparison, you can streamline your object querying and data manipulation operations. The examples ranged from straightforward findById usage to sophisticated custom schema type applications, offering scalability in your architectural decisions. Remember to treat Object IDs with attentiveness in your relational data schemes and optimized searching algorithms.