How to fix OverwriteModelError: Cannot overwrite model once compiled in MongoDB

MongoDBBEGINNERMEDIUM

This Mongoose error occurs when trying to register a model with the same name multiple times. It commonly happens in development environments with hot-reloading, serverless functions, or test suites that re-import model files.

What this error means

This error is thrown by Mongoose (the MongoDB ODM) when you attempt to compile a model that has already been registered in the Mongoose instance. Mongoose maintains a global registry of all compiled models in `mongoose.models`, and once a model is compiled with a specific name, it cannot be overwritten without clearing the registry first. The error typically occurs because Mongoose is designed to prevent accidental model redefinition, which could lead to inconsistent schema definitions and unpredictable behavior. When your application tries to call `mongoose.model('User', userSchema)` twice, Mongoose detects the duplicate registration and throws this error to prevent conflicts. This is particularly common in Next.js API routes (which are deployed as serverless functions), development environments with hot module replacement (HMR), test suites that reload models before each test, and applications that import model files multiple times through different paths.

How to fix "OverwriteModelError: Cannot overwrite model once compiled"

1Add conditional model export pattern

The safest solution is to check if the model already exists before creating it. Replace your model export with:

javascript

// models/User.js
const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  email: String,
  // ... other fields
});

// Check if model exists, otherwise create it
module.exports = mongoose.models.User || mongoose.model('User', userSchema);

For TypeScript/ES modules:

typescript

// models/User.ts
import mongoose from 'mongoose';

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

export default mongoose.models.User || mongoose.model('User', userSchema);

This pattern checks mongoose.models.User first and only creates the model if it doesn't exist.

2Ensure single model definition file

Make sure each model is defined and exported in exactly one file. Avoid duplicating model definitions across multiple files.

Good structure:

bash

models/
  ├── User.js
  ├── Post.js
  └── Comment.js

Import models consistently:

javascript

// Always import from the same location
const User = require('./models/User');
const Post = require('./models/Post');

Node.js caches required modules, so importing the same file multiple times will return the cached export.

3Fix test suite model imports

If you're getting this error in tests, avoid re-importing models in test hooks. Instead, import once at the top of the test file:

Before (causes error):

javascript

describe('User tests', () => {
  let User;

  beforeEach(() => {
    // DON'T: This re-imports and tries to recompile the model
    User = require('../models/User');
  });
});

After (fixed):

javascript

const User = require('../models/User'); // Import once at top

describe('User tests', () => {
  beforeEach(async () => {
    // Just clear data, don't re-import model
    await User.deleteMany({});
  });
});

4Clear Mongoose connection in tests (if needed)

If you need to completely reset Mongoose between test suites, properly disconnect and clear models:

javascript

// test/setup.js or afterAll hook
afterAll(async () => {
  // Clear all models from Mongoose registry
  mongoose.connection.models = {};

  // Close connection
  await mongoose.disconnect();
});

Or use Mongoose's deleteModel() method:

javascript

// Clear specific model
mongoose.deleteModel('User');

// Then you can safely recompile
const User = mongoose.model('User', userSchema);

How to fix OverwriteModelError: Cannot overwrite model once compiled in MongoDB

What this error means

Typical symptoms

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How to fix "OverwriteModelError: Cannot overwrite model once compiled"

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