Using Mixed Schema Type in Mongoose

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


Mongoose is a widely-used ODM (Object Document Mapping) library for MongoDB in Node.js applications. It enables developers to define schemas for their collections, which helps in maintaining some structure in a database that is schema-less by nature. While offering predefined schema types like String, Number, and Boolean, Mongoose also provides a special type known as ‘Mixed’ through the Schema.Types.Mixed data type. This type is akin to a wildcard, allowing you to store any valid BSON data-type in a given document path. Using Mixed type can be beneficial but also tricky due to its flexibility and less rigid structure. This article will explore real-world use-cases and best practices to effectively use the Mixed schema type, backed with code examples from basic to advanced scenarios.

Basic Example of Mixed Type in Mongoose

Before diving into advanced use cases, let’s discuss a basic example. The Mixed type is useful when the shape of your data cannot be strictly defined. Here’s how to include a Mixed type in your schema:

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

const anySchema = new Schema({
  arbitraryData: Schema.Types.Mixed

const AnyModel = mongoose.model('Any', anySchema);

This sets up an Any model which can take any structure under the arbitraryData field. Bear in mind that overusing Mixed type can lead to hard-to-maintain collections as the stored data lacks a consistent structure.

Handling Mixed Types with Structured Discipline

Even though the Mixed type can handle anything, discipline in its usage is crucial. Let’s enhance our model with some controlled, reading and writing logic in case of updates:

const controlledAnySchema = new Schema({
  metadata: Schema.Types.Mixed

controlledAnySchema.pre('save', function (next) {
  if (!this.isModified('metadata')) {
    return next();

const ControlledAnyModel = mongoose.model('ControlledAny', controlledAnySchema);

async function updateMetadata(docId, metadata) {
  await ControlledAnyModel.findByIdAndUpdate(docId, { $set: { metadata } }).exec();

This pattern includes a schema pre-save hook that tracks when the Mixed type field is modified. It emphasizes that you should explicitly signal Mongoose about such changes using the markModified(path) method for proper persistence.

The updateMetadata function is used to update the Mixed type field in an existing document, adhering to the async/await pattern for Promises.

Advanced Usage with Conditional Structures

In more advanced scenarios, the content of a Mixed type field might depend on other fields within the document. The following showcases how to define a schema where the Mixed type can vary based on another property:

const productSchema = new Schema({
  type: {
    type: String,
    required: true
  details: Schema.Types.Mixed

productSchema.pre('validate', function (next) {
  if (this.type === 'book' && typeof this.details !== 'object') {
    throw new Error('Details must be an object for book type products.');
  // Add further conditions for other product types if necessary

const Product = mongoose.model('Product', productSchema);

TThe example demonstrates using a dedicated mongoose hook, this time ‘validate’, to perform conditional checks before persisting the document. This leverages the flexibility of Mixed type while still keeping some level of structural integrity based on certain conditions.

Interoperability of Mixed Type with TS/ES Modules

When using TypeScript or modern JavaScript ES modules, you can also handle Mongoose schemas effectively. Here’s a quick demonstration:

import mongoose, { Schema } from 'mongoose';\n
interface IMetadata {
  [key: string]: any;\n}

interface IAny extends mongoose.Document {
  metadata: IMetadata;\n}

const anyWithTS: Schema = new mongoose.Schema({\n  metadata: {\n    type: Schema.Types.Mixed,\n    required: true\n  }\n});

const AnyModelWithTS = mongoose.model('AnyWithTS', anyWithTS);

// Using this model now will take advantage of TypeScript's typing\nasync function addMetadata(masterId: string, metadata: IMetadata) {\n  const docInstance = new AnyModelWithTS({ metadata });\n  await;\n}

This example illustrates how TypeScript interfaces integrate well with the flexibility of Mixedtypes in Mongoose. Note the structured approach towards unstructured data, still allowing the developer to apply any set of key-value pairs as metadata.


The Mixedschema type is a powerful feature in Mongoose that should be used with careful consideration. It provides complete flexibility in terms of what you can store in document fields but should be tempered with checks, validations, and organizational nuances to prevent future headaches. Through careful application and responsible usage, the Mixed type can be an excellent tool for accommodating evolving data requirements or handling loosely-structured data within your Node.js, TypeScript, or ES6 applications powered by MongoDB.

When effectively managed, it affords your collections the ability to evolve over time, adapt to different needs, and interoperate with diverse structures in MongoDB. Combining Mixedhewith rigorous validation and update strategies equipped with the modern syntax of TypeScript or ES modules ensures maintainability and scalability of your Node.js applications. Always weigh the pros and cons of schema flexibility and enforce document structure whenever possible to keep your data both accommodating and reliable.