Version: 4.2


Under the hood, MikroORM uses ts-morph to read TypeScript source files of all entities to be able to detect all types. Thanks to this, defining the type is enough for runtime validation.

This has some consequences for deployment of your application. Sometimes you will want to deploy only your compiled output, without TS source files at all. In that case, discovery process will probably fail. You have several options:

Deploy pre-built cache#

By default, output of metadata discovery will be cached in temp folder. You can reuse this cache in your deployed application. Currently the cache is saved in files named like the entity source file, e.g. Author.ts entity will store cache in temp/Author.ts.json file.

When running compiled code, JS entities will be taken into account instead, so you will need to generate the cache by running the compiled code locally. That will generate temp/Author.js.json, which is the file you will need to deploy alongside your application.

Fill type or entity attributes everywhere#

What discovery process does is to sniff TS types and save their value to string, so it can be used later for validation. You can skip the whole process by simply providing those values manually:

export class Book {
@PrimaryKey({ type: 'number' })
id!: number;
@Property({ type: 'string' })
title!: string;
@Enum(() => BookStatus)
status?: BookStatus;
@ManyToOne(() => Author) // or `@ManyToOne({ type: 'Author' })` or `@ManyToOne({ entity: () => Author })`
author1!: Author;
// or
@ManyToOne({ type: 'Author' })
author2!: Author;
// or
@ManyToOne({ entity: () => Author })
author3!: Author;
export enum BookStatus {
SOLD_OUT = 'sold',
ACTIVE = 'active',
UPCOMING = 'upcoming',

For numeric enums this is not be required.

Deploy your entity source files#

Usually it does not matter much that you deploy more files than needed, so the easiest way is to just deploy your TS source files next to the compiled output, just like during development.

Deploy a bundle of entities and dependencies with Webpack#

Webpack can be used to bundle every entity and dependency: you get a single file that contains every required module/file and has no external dependencies.

Prepare your project for Webpack#

Webpack requires every required file to be hardcoded in your code. Code like this won't work (it will throw an error because Webpack doesn't know which file to include in the bundle):

let dependencyNameInVariable = 'dependency';
const dependency = import(dependencyNameInVariable);

As Webpack creates a file bundle, it isn't desired that it scans directories for entities or metadata. Therefore you need to provide list of entities in the entities option in the initialization function, folder/file based discovery is not supported (see dynamically including entities as an alternative solution). Also you need to fill type or entity attributes everywhere (see above) and disable caching (it will decrease start-time slightly).

In v4 caching is disabled by default when using ReflectMetadataProvider.

Disabling dynamic file access#

First thing you should do is to disable dynamic file access in the discovery process via the discovery.disableDynamicFileAccess toggle. This will effectively do:

  • set metadata provider to ReflectMetadataProvider
  • disable caching
  • disallow usage of paths in entities/entitiesTs

Manually defining entities#

import { Author, Book, BookTag, Publisher, Test } from '../entities';
await MikroORM.init({
entities: [Author, Book, BookTag, Publisher, Test],
discovery: { disableDynamicFileAccess: true },

Dynamically loading dependencies#

This will make use of a Webpack feature called dynamic imports. This way you can import dependencies as long as part of the path is known.

In following example require.context is used. This 'function' is only usable during the building process from Webpack so therefore there is an alternative solution provided that will as long as the environment variable WEBPACK is not set (e.g. during development with ts-node).

Here, all files with the extension .ts will be imported from the directory ../entities.

flatMap is a method from ECMAScript 2019 and requires Node.js 11 or higher.

await MikroORM.init({
// ...
entities: await getEntities(),
discovery: { disableDynamicFileAccess: true },
// ...
async function getEntities(): Promise<any[]> {
if (process.env.WEBPACK) {
const modules = require.context('../entities', true, /\.ts$/);
return modules
.map(r => modules(r))
.flatMap(mod => Object.keys(mod).map(className => mod[className]));
const promises = fs.readdirSync('../entities').map(file => import(`../entities/${file}`));
const modules = await Promise.all(promises);
return modules.flatMap(mod => Object.keys(mod).map(className => mod[className]));

Webpack configuration#

Webpack can be run without configuration file but for building MikroORM and Node.js bundles it requires additional configuration. Configuration for Webpack is stored in the root of the project as webpack.config.js. For all the options please refer to the following page.

For bundling MikroORM the following configuration is required:

const path = require('path');
const { EnvironmentPlugin, IgnorePlugin } = require('webpack');
const TerserPlugin = require('terser-webpack-plugin');
// Mark our dev dependencies as externals so they don't get included in the webpack bundle.
const { devDependencies } = require('./package.json');
const externals = {};
for (const devDependency of Object.keys(devDependencies)) {
externals[devDependency] = `commonjs ${devDependency}`;
// And anything MikroORM's packaging can be ignored if it's not on disk.
// Later we check these dynamically and tell webpack to ignore the ones we don't have.
const optionalModules = new Set([
...Object.keys(require('@mikro-orm/core/package.json').devDependencies || {})
module.exports = {
entry: path.resolve('app', 'server.ts'),
// You can toggle development mode on to better see what's going on in the webpack bundle,
// but for anything that is getting deployed, you should use 'production'.
// mode: 'development',
mode: 'production',
optimization: {
minimizer: [
new TerserPlugin({
terserOptions: {
// We want to minify the bundle, but don't want Terser to change the names of our entity
// classes. This can be controlled in a more granular way if needed, (see
// but the safest default
// config is that we simply disable mangling altogether but allow minification to proceed.
mangle: false,
// Similarly, Terser's compression may at its own discretion change function and class names.
// While it only rarely does so, it's safest to also disable changing their names here.
// This can be controlled in a more granular way if needed (see
compress: {
keep_classnames: true,
keep_fnames: true,
target: 'node',
module: {
rules: [
// Bring in our typescript files.
test: /\.ts$/,
exclude: /node_modules/,
loader: 'ts-loader',
// Native modules can be bundled as well.
test: /\.node$/,
use: 'node-loader',
// Some of MikroORM's dependencies use mjs files, so let's set them up here.
test: /\.mjs$/,
include: /node_modules/,
type: 'javascript/auto',
// These are computed above.
resolve: {
extensions: ['.ts', '.js']
plugins: [
// Ignore any of our optional modules if they aren't installed. This ignores database drivers
// that we aren't using for example.
new EnvironmentPlugin({ WEBPACK: true }),
new IgnorePlugin({
checkResource: resource => {
const baseResource = resource.split("/", resource[0] === "@" ? 2 : 1).join("/");
if (optionalModules.has(baseResource)) {
try {
return false;
} catch {
return true;
return false;
output: {
filename: 'server.js',
libraryTarget: 'commonjs',
path: path.resolve(__dirname, '..', 'output'),

Running Webpack#

To run Webpack execute webpack (or npx webpack if not installed globally) in the root of the project. It will probably throw a few warnings but you can ignore the errors regarding MikroORM: the mentioned pieces of code won't be executed if properly bundled with Webpack.

Last updated on by Martin Adámek