MONGOOSE LEAN EXTENSION

A powerful Mongoose plugin that extends the functionality of .lean() queries to eliminate common post-processing steps — like stringifying ObjectIds, renaming _id key to something else, and removing internal Mongoose metadata like __v.

---

mongoose-lean-extension@1.0.0 is here. Welcome this major version update. 0 -> 1

Breaking Changes:

Read through this README for complete overview Visit my GitHub repo for the source code Typescript support My contact are everywhere in this source code

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Overview

Mongoose’s .lean() is a great performance optimization, returning plain JavaScript objects instead of full Mongoose documents. But it comes with annoyances:

• ObjectId keys are still BSON types (ObjectId) that require manual conversion.
• The __v version key is often unnecessarily present in results.
• Deeply nested ObjectIds in subdocuments aren't stringified without additional effort.

mongoose-lean-extension solves these problems by automating the transformation of .lean() results. It enables:

• Auto-stringification of top-level _id fields
• Auto-removal of the __v field
• Optional stringification of nested ObjectId fields via path expressions (e.g. contributors._id)

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Installation

npm i mongoose-lean-extension

🔧 Usage

Global Plugin (Recommended)

const mongooseLeanExtension = require("mongoose-lean-extension");
const mongoose = require("mongoose");

// Apply plugin globally to all schemas
mongoose.plugin(mongooseLeanExtension);

Per-Schema Usage

const mongooseLeanExtension = require("mongoose-lean-extension");

const schema = new mongoose.Schema({
    /* ... */
});
schema.plugin(mongooseLeanExtension);

IMPORTANT NOTICE

• All plugins are attached to the schema before it is compiled to create a model

  ⚡ mongoose-lean-extension introduces negligible latency (<2ms per 100 documents) on average .lean() queries.

• For how utility function in stringifyKeys

  See util/stringifyPaths.js

Configuration Options

The plugin is configured via the .lean() query method.

Model.find().lean({
    stringifyId: true, // (default: true) — Convert `_id` to string
    showVersion: false, // (default: false) — Remove `__v` from results
    stringifyKeys: ["nested._id"], // (optional) — Array of mongo-like paths [dot-paths] to nested ObjectId fields to stringify
    rename: "key", // Renames `_id` to `key`
});

Option Details

Option	Type	Default	Description
stringifyId	boolean	true	If true, stringifies the document _id. If false, leaves as ObjectId.
showVersion	boolean	false	If false, removes the __v field from results if they exist otherwise, leaves them.
stringifyKeys	string[]	[]	Mongo-like (Dot-separated) paths to any nested ObjectId fields to convert into strings.
rename	string	undefined	Renames _id to provided string value.

Example

const mongooseLeanExtension = require("mongoose-lean-extension");
const mongoose = require("mongoose");

mongoose.plugin(mongooseLeanExtension);

const ContributorSchema = new mongoose.Schema({
    username: { type: String, required: true },
    languages: [String],
});

const PackageSchema = new mongoose.Schema(
    {
        name: { type: String, required: true },
        contributors: [ContributorSchema], // This will have ObjectId _id for each of the nested document, Perfect for nested stringification
    },
    { timestamps: true }
);

const Package = mongoose.model("Package", PackageSchema);

/**
 * await Package.insertMany([
 *    {
 *        name: "express",
 *        contributors: [
 *            {
 *              username: "tjholowaychuk",
 *              languages: ["JavaScript", "TypeScript"],
 *            },
 *            {
 *              username: "dougwilson",
 *              languages: ["JavaScript"],
 *            },
 *        ],
 *    },
 *    {
 *      name: "react",
 *      contributors: [
 *          {
 *             username: "gaearon",
 *             languages: ["JavaScript", "TypeScript"],
 *          },
 *          {
 *            username: "bvaughn",
 *            languages: ["JavaScript"],
 *          },
 *      ],
 *    }
 * ])
 */

const packages = await Package.find().lean({
    stringifyKeys: ["contributors._id"],
});

console.dir(packages, { depth: null });

Output

[
  {
    "_id": "683a2480b53cfe150c05c5a8",
    "name": "express",
    "contributors": [
      {
        "username": "tjholowaychuk",
        "languages": ["JavaScript", "TypeScript"],
        "_id": "683a2480b53cfe150c05c5a9"
      },
      ...
    ],
    "createdAt": "2025-05-30T21:34:56.981Z",
    "updatedAt": "2025-05-30T21:34:56.981Z"
  },
  ...
]

How It Works

• Under the hood, the plugin attaches a post-query middleware on find and findOne:

• If the query uses .lean(), it inspects custom plugin options from lean({...}).

• _id fields are converted to .toString() if enabled.

• __v is stripped if disabled.

Nested ObjectIds are stringified recursively using a safe traversal method.

Custom Path Stringification

The helper function applyStringifyAtPath(object, path) walks through nested objects and arrays to convert specific paths like:

stringifyKeys: ["contributors._id", "authors.contact._id"];

Plugin Architecture

This package consists of a modular design that allows importing specific plugins if needed.

Included Plugins

Plugin	File	Purpose
stringifyId	plugins.js	Converts top-level _id fields
rename	plugins.js	Renames top-level _id to provided string value
deversion	plugins.js	Removes __v fields
stringifyKeys	plugins.js	Stringifies custom ObjectId paths
main	index.js	All-in-one plugin (recommended)

Example: Using Only deversion

const { deversion } = require("mongoose-lean-extension/plugins");
mongoose.plugin(deversion);

Example: Selective Composition

const { stringifyId } = require("mongoose-lean-extension/plugins.js");
const { stringifyKeys } = require("mongoose-lean-extension/plugins.js");
const { deversion } = require("mongoose-lean-extension/plugins.js");
const { rename } = require("mongoose-lean-extension/plugins.js");

const schema = new mongoose.Schema({...});
schema.plugin(stringifyKeys);
schema.plugin(stringifyId);
schema.plugin(deversion);
schema.plugin(rename);

Testing

Unit tests are written using Jest.

$ npm run test

> mongoose-lean-extension@0.1.3 test
> jest --coverage

 PASS  __tests__/index.test.js
 PASS  __tests__/plugins.test.js (5.995 s)
------------------------------|---------|----------|---------|---------|-------------------
File                          | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
------------------------------|---------|----------|---------|---------|-------------------
All files                     |   95.91 |    86.66 |     100 |   97.64 |
 Mongoose Lean Extension      |   95.23 |    84.81 |     100 |   97.22 |
  index.js                    |   93.33 |    87.09 |     100 |   96.29 | 61
  plugins.js                  |   96.29 |    83.33 |     100 |   97.77 | 59
 Mongoose Lean Extension/util |     100 |      100 |     100 |     100 |
  stringifyPaths.js           |     100 |      100 |     100 |     100 |
------------------------------|---------|----------|---------|---------|-------------------

Test Suites: 2 passed, 2 total
Tests:       29 passed, 29 total
Snapshots:   0 total
Time:        6.591 s
Ran all test suites.

Includes tests using mongodb-memory-server for in-memory database mocking.

Advanced

How to Preserve _id as ObjectId?

Model.find().lean({ _id: false }); // Disables stringification of `_id`

Or; do not use the all-in-one plugin at "global level" rather use the purpose-specific at "schema level"

How to Keep __v?

Model.find().lean({ __v: true }); // Keeps __v field in results

Or; do not use the all-in-one plugin at "global level" rather use the purpose-specific at "schema level"

How to Keep nested ObjectId fields?

Model.find().lean(); // Keeps nested ObjectIds

Or; do not use the plugin

Extras

async function __run_() {
    await mongoose.connect("mongodb://localhost:27017/trials");

    const User = mongoose.model("User", new Schema({ name: String, age: Number }));
    const Car = mongoose.model(
        "Car",
        new Schema({
            model: String,
            owner: { type: mongoose.Schema.Types.ObjectId, ref: "User" },
        })
    );

    const ContributorSchema = new Schema({
        username: { type: String, required: true },
        languages: [String],
    });
    const PackageSchema = new Schema(
        {
            name: { type: String, required: true },
            contributors: [ContributorSchema],
        },
        { timestamps: true }
    );

    /** First time? Un-comment the lines below and run the code.
     * After initial run comment them again (or delete)
     */

    // await Car.deleteMany({});
    // await User.deleteMany({});

    // await Promise.allSettled([
    //     User.create({ name: "Ariella", age: 2 }),
    //     User.create({ name: "Arianna", age: 33 }),
    // ]);

    // await Promise.allSettled([
    //     Car.create({
    //         model: "BMW",
    //         owner: Types.ObjectId.createFromHexString("6839968512bb0d7b0d62e23e"),
    //     }),
    //     Car.create({
    //         model: "Rover",
    //         owner: Types.ObjectId.createFromHexString("6839968512bb0d7b0d62e23e"),
    //     }),
    //     Car.create({
    //         model: "Toyota",
    //         owner: Types.ObjectId.createFromHexString("6839968512bb0d7b0d62e23f"),
    //     }),
    // ]);

    const users = await User.find().lean();
    console.log(users);

    const cars = await Car.find().populate("owner").lean(); // if "stringifyId" plugin is used on mongoose "global level" that is, mongoose.plugin(stringifyId) and .populate("field") is on the query, both car._id and car.owner._id will be hex strings however, if it is used on schema level, for example, CarSchema.plugin(stringifyId) only: car._id will be hex string but car.owner._id will be ObjectId... if UserSchema.plugin(stringifyId): car.owner._id will be hex string but car._id will be ObjectId.
    console.log(cars);

    /** cars.owner might be null in case you do not assign the right object ids as new ones are created everytime a document is recorded. Check "owner" property assigned value for await Promise.allSettled([ ... ]) for Car.create({}) */

    await mongoose.disconnect();
}

__run_().catch(console.error);

✅ Compatibility

• ✅ Mongoose 8.x
• ✅ MongoDB >= 4.x

Contributing

Pull requests are welcome! Feel free to open issues or suggestions on improvements, use cases, or performance enhancements.

Feedback

If this plugin saves you time or solves your pain, please consider starring the repo and sharing it with the Mongoose community!

Let me know if you'd like this split into multiple files (e.g. for GitHub pages) or want me to generate a live demo repo or documentation website from it.

License

This project is licensed under the MIT License - see the LICENSE file for details.

Contributors

1. Ssekandi - Author

😏 Gotcha Section {Ignore if you are in a bad mood}

• Aaah nothing much, just weekly downloads reduce when I do not update this package.
• Anyways, I added the rename functionality because I found it a little awkward to use _id in my data for React Native's FlatList keyExtractor prop.