Skip to main content
Version: 6.1

Chapter 1: First Entity

Setting up

Before we start, ensure you meet the following pre-requisites first:

  1. Have Node.js version 18.12 or higher installed, but preferably version 20.
  2. Have NPM installed, or use any other package manager of your choice.
    • NPM comes bundled with Node.js, so you should already have it. If not, reinstall Node.js. To use other package managers, consider using corepack.

If not certain, confirm the prerequisites by running:

node -v
npm -v

Creating a new project

Let's start with the basic folder structure. As we said we will have 3 modules, each having its own directory:

# create the project folder and `cd` into it
mkdir blog-api && cd blog-api
# create module folders, inside `src` folder
mkdir -p src/modules/{user,article,common}

Now add the dependencies:

npm install @mikro-orm/core \
@mikro-orm/sqlite \
@mikro-orm/reflection \

And some development dependencies:

npm install --save-dev @mikro-orm/cli \
typescript \
ts-node \
@types/node \

ECMAScript Modules

You probably heard about ECMAScript Modules (ESM), but this might easily be the first time you try them. Now keep in mind - the whole ecosystem is far away from ready, and this guide is using the ESM mainly to show how it is possible. Many dependencies are not ESM ready, and often there are weird workarounds needed. MikroORM is no exception to this - there are quirks as well, namely in dynamic imports of TypeScript files under test setup.

In a nutshell, for ESM project we need to:

  • add "type": "module" to package.json
  • use import/export statements instead of require calls
  • use .js extension in those imports, even in TypeScript files
  • configure TypeScript and ts-node property, as described in the following section

You can read more about the ESM support in Node.js here.

In addition to this, there is one gotcha with defining entities using decorators. The default way MikroORM uses to obtain a property type is via reflect-metadata. While this itself introduces some challenges and limitations, we can't use it in an ESM project. This is because, with ES modules, the dependencies are resolved asynchronously, in parallel, which is incompatible with how the reflect-metadata module currently works. For this reason, we need to use other ways to define the entity metadata - in this guide, we will use the @mikro-orm/reflection package, which uses ts-morph under the hood to gain information from TypeScript Compiler API. This works fine with ESM projects, and also opens up new ways of compiling the TypeScript files, like esbuild (which does not support decorator metadata).

Another way to define your entities is via EntitySchema, this approach works also for vanilla JavaScript projects, as well as allows to define entities via interfaces instead of classes. Check the Defining Entities section, all examples there have code tabs with definitions via EntitySchema too.

The reflection with ts-morph is performance heavy, so the metadata are cached into temp folder and invalidated automatically when you change your entity definition (or update the ORM version). You should add this folder to .gitignore file. Note that when you build your production bundle, you can leverage the CLI to generate production cache on build time to get faster start-up times. See the deployment section for more about this.

Configuring TypeScript

We will use the following TypeScript config, so create the tsconfig.json file and copy it there. If you know what you are doing, you can adjust the configuration to fit your needs.

For ESM support to work, we need to set module and moduleResolution to Node16 and target ES2022. We also enable strict mode and experimentalDecorators, as well as the declaration option to generate the .d.ts files, needed by the @mikro-orm/reflection package. Lastly, we tell TypeScript to compile into dist folder via outDir and make it include all *.ts files inside src folder.

"compilerOptions": {
"module": "Node16",
"target": "ES2022",
"strict": true,
"outDir": "dist",
"declaration": true,
"experimentalDecorators": true
"include": [

Using ts-node to run TypeScript files directly

During the development, you will often want to run the app to test your changes. For that, it is handy to use ts-node to run the TypeScript files directly, instead of compiling the first to JavaScript. By default, ts-node will operate in CommonJS mode, so we need to configure it via tsconfig.json to enable ESM support. To speed things up, we can use the transpileOnly option, which disables type checking - we will get the type checks when compiling the app for production use, so it does not matter that much during development (especially when you have an IDE that shows you the errors anyway).

"compilerOptions": { ... },
"include": [ ... ],
"ts-node": {
"esm": true,
"transpileOnly": true

Setting up CLI

Next, we will set up the CLI config MikroORM. This config will be then automatically imported into your app too. Define the config variable with explicit Options type, that way you get the best level of type safety - autocomplete as well as warning about not existing options (as opposed to using { ... } as Options, that won't warn you for such).

For tests, you can import the config and override some options before evaluating it.

import { Options, SqliteDriver } from '@mikro-orm/sqlite';
import { TsMorphMetadataProvider } from '@mikro-orm/reflection';

const config: Options = {
// for simplicity, we use the SQLite database, as it's available pretty much everywhere
driver: SqliteDriver,
dbName: 'sqlite.db',
// folder-based discovery setup, using common filename suffix
entities: ['dist/**/*.entity.js'],
entitiesTs: ['src/**/*.entity.ts'],
// we will use the ts-morph reflection, an alternative to the default reflect-metadata provider
// check the documentation for their differences:
metadataProvider: TsMorphMetadataProvider,
// enable debug mode to log SQL queries and discovery information
debug: true,

export default config;

Note that we are importing Options from the @mikro-orm/sqlite package - this is an alias to Options<SqliteDriver>.

Alternatively, we can use the defineConfig helper that should provide intellisense even in JavaScript files, without the need for type hints:

import { defineConfig } from '@mikro-orm/sqlite';
import { TsMorphMetadataProvider } from '@mikro-orm/reflection';

// no need to specify the `driver` now, it will be inferred automatically
export default defineConfig({
dbName: 'sqlite.db',
// folder-based discovery setup, using common filename suffix
entities: ['dist/**/*.entity.js'],
entitiesTs: ['src/**/*.entity.ts'],
// we will use the ts-morph reflection, an alternative to the default reflect-metadata provider
// check the documentation for their differences:
metadataProvider: TsMorphMetadataProvider,
// enable debug mode to log SQL queries and discovery information
debug: true,

Save this file into src/mikro-orm.config.ts, so it will get compiled together with the rest of your app. Next, you need to tell the ORM to enable TypeScript support for CLI, via mikro-orm section in the package.json file.

Alternatively, you can use mikro-orm.config.js file in the root of your project, such a file will get loaded automatically. Consult the documentation for more info.

"type": "module",
"dependencies": { ... },
"devDependencies": { ... },
"mikro-orm": {
"useTsNode": true

The useTsNode option here tells the CLI to automatically enable TypeScript support, otherwise our .ts config file would be ignored. This option is only for the CLI, when you run your app, it is your responsibility to bring the runtime TypeScript support if you want to (e.g. via ts-node as described in this guide).

Lastly, add some NPM scripts to ease the development. We will build the app via tsc, test it via vitest and run it locally via ts-node. There is one gotcha with ESM and dynamic imports. While it works fine for regular JavaScript files, once we mix runtime support for TypeScript via ts-node or vitest/esbuild, you start hitting the wall with errors like Unknown file extension ".ts". To get around that, we can use the ts-node/esm loader via NODE_OPTIONS environment variable - but that can get ugly, and we can do better - at least for the CLI, we have the mikro-orm-esm script, which automatically registers the ts-node/esm loader as well as disables the experimental warning.

So remember - always use mikro-orm-esm in the ESM projects with TypeScript. Note that it requires the ts-node dependency to be installed, if you don't use TypeScript, the regular mikro-orm script will work fine for you.

This issue with dynamic imports can surface for both the CLI usage and vitest. While the --loader solution works for the CLI, we can use something more vite-native for vitest, let's talk about that part later when you start writing the first test.

The ts-node binary works only on older Node.js versions, for v20 or above, we need to use node --loader ts-node/esm instead.

"type": "module",
"dependencies": { ... },
"devDependencies": { ... },
"mikro-orm": { ... },
"scripts": {
"build": "tsc",
"start": "node --no-warnings=ExperimentalWarning --loader ts-node/esm src/server.ts",
"test": "vitest"

We refer to a file src/server.ts in the start script, we will create that later, no need to worry about it right now.

Now test the CLI via npx mikro-orm-esm debug, you should see something like the following:

Current MikroORM CLI configuration
- dependencies:
- mikro-orm 6.0.0
- node 20.9.0
- knex 3.0.1
- sqlite3 5.1.6
- typescript 5.3.3
- package.json found
- ts-node enabled
- searched config paths:
- /blog-api/src/mikro-orm.config.ts (found)
- /blog-api/dist/mikro-orm.config.js (found)
- /blog-api/mikro-orm.config.ts (not found)
- /blog-api/mikro-orm.config.js (not found)
- configuration found
- database connection successful
- will use `entities` array (contains 0 references and 1 paths)
- /blog-api/dist/**/*.entity.js (not found)
- could use `entitiesTs` array (contains 0 references and 1 paths)
- /blog-api/src/**/*.entity.ts (not found)

This looks good, we get a nice summary of what is being installed, we can see the config being loaded correctly, and as expected, no entities were discovered - because you need to create them first!

If you used npx mikro-orm debug instead of npx mikro-orm-esm debug, the configuration would fail to be loaded and an error similar to this one would be present:

Unknown file extension ".ts" for ./blog-api/src/mikro-orm.config.ts

Then test the TypeScript build, as we now have the first file we can compile. Use npm run build and check if the dist folder gets generated with the JavaScript version of our config file.

Before we get to creating the very first entity, let's do a quick sanity check - this is our initial directory structure so far:

├── dist
│ └── mikro-orm.config.js
├── node_modules
├── package-lock.json
├── package.json
├── src
│ ├── mikro-orm.config.ts
│ └── modules
│ ├── article
│ ├── common
│ └── user
└── tsconfig.json

First Entity

This was quite a lot of setup, but don't worry, most of the heavy lifting is behind you. Time to create your first entity - the User! Create a user.entity.ts file in src/modules/user with the following contents:


Check out the [Defining Entities section which provides many examples of various property types as well as different ways to define your entities.

import { Entity, PrimaryKey, Property } from '@mikro-orm/core';

export class User {

id!: number;

fullName!: string;

email!: string;

password!: string;

@Property({ type: 'text' })
bio = '';


So what do we have here? An entity is a JavaScript class, decorated with an @Entity() decorator, that defines properties with other decorators (like @Property()). An entity represents a database table, and the properties represent its columns.

We use the *.entity.ts suffix for easy folder-based discovery across module boundaries. Alternatively, you could explicitly provide the entity class references in the ORM config, e.g. entities: [User]. With the explicit setup, things are more streamlined and less error-prone, there is no dynamic importing, and no file system is involved. But folder-based discovery can be handy, especially when our app grows to many entities.

Defining the primary key

Every entity needs to have a primary key, we will use a simple auto-increment numeric one. MikroORM defaults to that when it sees a single primary key property with a number type, so doing the following is enough:

id!: number;

In case you want to use bigint column type, just pass type: 'bigint' in the decorator options. BigInts are mapped to string, as they would not fit into JavaScript number safely.

@PrimaryKey({ type: 'bigint' })
id!: string;

Another common use case is UUID. We can leverage the fact, that MikroORM never calls your entity constructor when creating managed entity instances (those loaded from your database). This means property initializers (or in general constructors) are executed only for entities that will produce an INSERT query.

@PrimaryKey({ type: 'uuid' })
uuid = uuid.v4();

Scalar properties

To map regular database columns we can use the @Property() decorator. It works the same as the @PrimaryKey() decorator describer above. You could say it extends it - all the properties you can pass to the @Property() decorator are also available in @PrimaryKey() too.

We are using the ts-morph metadata provider, which helps with advanced type inference. Check out the documentation for the differences if you'd like to use the default metadata provider which is based on reflect-metadata.

The ORM will automatically map string properties to varchar, for the we want to use text instead, so we change it via the type decorator option:

@Property({ type: 'text' })
bio = '';

The type option here allows several input forms. We are using the text type name here, which is mapped to the TextType - a mapped type representation used internally by the ORM. If you provide a string value there, and it won't match any known type alias, it will be considered as the column type. We can also provide a type class implementation instead of a string type name:

import { TextType } from '@mikro-orm/core';

@Property({ type: TextType })
bio = '';

There is also a types map (exported also as just t for brevity):

import { t } from '@mikro-orm/core'; // `t` or `types`

@Property({ type: t.text })
bio = '';

We can always provide the explicit column type (and this can be even combined with the type option, to override how the mapped type is implemented):

import { t } from '@mikro-orm/core'; // `t` or `types`

@Property({ columnType: 'character varying(1000)' })
bio = '';

When using the columnType, be careful about options like length or precision/scale - columnType is always used as-is, without any modification. This means you need to pass the final value there, including the length, e.g. columnType: 'decimal(10,2)'.

Working with Entity Manager

So now you have the access to EntityManager, let's talk about how it works and how you can use it.

Persist and Flush

There are 2 methods we should first describe to understand how persisting works in MikroORM: em.persist() and em.flush().

em.persist(entity) is used to mark new entities for future persisting. It will make the entity managed by the EntityManager and once flush will be called, it will be written to the database.

const user = new User(); = '';

// first mark the entity with `persist()`, then `flush()`
await em.flush();

// we could as well use fluent API here and do this:
await em.persist(user).flush();

To understand flush, let's first define what managed entity is: An entity is managed if it's fetched from the database (via em.find()) or registered as new through em.persist() and flushed later (only after the flush it becomes managed).

em.flush() will go through all managed entities, compute appropriate change sets and perform according database queries. As an entity loaded from the database becomes managed automatically, we do not have to call persist on those, and flush is enough to update them.

const user = await em.findOne(User, 1); = '...';

// no need to persist `user` as it's already managed by the EM
await em.flush();

Let's try to create our first record in the database, add this to the server.ts file instead of the console.log:

// create new user entity instance
const user = new User(); = '';
user.fullName = 'Foo Bar';
user.password = '123456';

// first mark the entity with `persist()`, then `flush()`
await orm.em.persist(user).flush();

// after the entity is flushed, it becomes managed, and has the PK available
console.log('user id is:',;

Now run the script again via npm start, and you will see an error:

ValidationError: Using global EntityManager instance methods for context specific actions is disallowed.
If you need to work with the global instance's identity map, use `allowGlobalContext` configuration option
or `fork()` instead.

Remember we said the orm.em is a global EntityManager instance? Looks like it is not a good idea to use it, in fact, it is disallowed by default. Before we get to the bottom of this message, let's quickly define two more terms we haven't touched yet - the Identity Map and Unit of Work.

  • Unit of Work maintains a list of objects (entities) affected by a business transaction and coordinates the writing out of changes.
  • Identity Map ensures that each object (entity) gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.

MikroORM is a data-mapper that tries to achieve persistence-ignorance. This means you map JavaScript objects into a relational database that doesn't necessarily know about the database at all. How does it work?

Unit of Work and Identity Map

MikroORM uses the Identity Map pattern to track objects. Whenever you fetch an object from the database, MikroORM will keep a reference to this object inside its UnitOfWork. This allows MikroORM room for optimizations. If you call the EntityManager and ask for an entity with a specific ID twice, it will return the same instance:

const jon1 = await em.findOne(Author, 1);
const jon2 = await em.findOne(Author, 1);

// identity map in action
console.log(jon1 === jon2); // true

The Identity Map only knows objects by id, so a query for different criteria has to go to the database, even if it was executed just before. But instead of creating a second Author object MikroORM first gets the primary key from the row and checks if it already has an object inside the UnitOfWork with that primary key.

Change Tracking

The identity map has a second, more important use-case. Whenever you call em.flush(), the ORM will iterate over the Identity Map, and for each entity it compares the original state with the values that are currently set on the entity. If changes are detected, the object is queued for an SQL UPDATE operation. Only the fields that changed are part of the update query.

The following code will update your database with the changes made to the Author object, even if you did not call em.persist():

const jon = await em.findOne(Author, 1); = '';

await em.flush();

Implicit Transactions

The most important implication of having Unit of Work is that it allows handling transactions automatically.

When you call em.flush(), all computed changes are queried inside a database transaction. This means that you can control the boundaries of transactions by calling em.persist() and once all your changes are ready, calling flush() will run them inside a transaction.

You can also control the transaction boundaries manually via em.transactional(cb).

const user = await em.findOne(User, 1); = '';
const car = new Car();;

await em.flush();

You can find more information about transactions in Transactions and concurrency page.

Why is Request Context needed?

Now back to the validation error about global context. With the freshly gained knowledge, we know EntityManager maintains a reference to all the managed entities in the Identity Map. Imagine we would use a single Identity Map throughout our application (so a single global context, global EntityManager). It will be shared across all request handlers, that can run in parallel.

  1. growing memory footprint

    As there would be only one shared Identity Map, we can't just clear it after our request ends. There can be another request working with it so clearing the Identity Map from one request could break other requests running in parallel. This will result in a growing memory footprint, as every entity that became managed at some point in time would be kept in the Identity Map.

  2. unstable response of API endpoints

    Every entity has toJSON() method, that automatically converts it to serialized form If we have only one shared Identity Map, the following situation may occur:

    Let's say there are 2 endpoints

    1. GET /article/:id that returns just the article, without populating anything
    2. GET /article-with-author/:id that returns the article and its author populated

    Now when someone requests the same article via both of those endpoints, we could end up with both returning the same output:

    1. GET /article/1 returns Article without populating its property author property
    2. GET /article-with-author/1 returns Article, this time with author populated
    3. GET /article/1 returns Article, but this time also with author populated

    This happens because the information about entity association being populated is stored in the Identity Map.

Fork to the win!

So we understand the problem better now, what's the solution? The error suggests it - forking. With the fork() method we get a clean EntityManager instance, that has a fresh Unit of Work with its own context and Identity Map.

// fork first to have a separate context
const em = orm.em.fork();

// first mark the entity with `persist()`, then `flush()`
await em.persist(user).flush();

Running npm start again, we get past the global context validation error, but only to find another one:

TableNotFoundException: insert into `user` (`bio`, `email`, `full_name`, `password`) values ('', '', 'Foo Bar', '123456') - no such table: user

We forgot to create the database schema. Fortunately, we have all the tools we need at hand. You can use the SchemaGenerator provided by MikroORM to create the schema, as well as to keep it in sync when you change your entities. For the initial testing, let's use the refreshDatabase() method, which is handy for testing - it will first drop the schema if it already exists and create it from scratch based on entity definition (metadata).

// recreate the database schema
await orm.schema.refreshDatabase();

Finally, npm start should succeed, and if you enabled the debug mode in your config, you will see the SQL queries in the logs, as well as the value at the very end.

[query] create table `user` (`id` integer not null primary key autoincrement, `full_name` text not null, `email` text not null, `password` text not null, `bio` text not null); [took 1 ms]
[query] pragma foreign_keys = on; [took 0 ms]
[query] begin
[query] insert into `user` (`bio`, `email`, `full_name`, `password`) values ('', '', 'Foo Bar', '123456') [took 0 ms]
[query] commit
user id is: 1

You can see the insert query being wrapped inside a transaction. That is another effect of the Unit of Work. The em.flush() call will perform all the queries inside a transaction. If something fails, the whole transaction will be rolled back.

Fetching Entities

We have our first entity stored in the database. To read it from there we can use find() and findOne() methods.

// find user by PK, same as `em.findOne(User, { id: 1 })`
const userById = await em.findOne(User, 1);
// find user by email
const userByEmail = await em.findOne(User, { email: '' });
// find all users
const allUsers = await em.find(User, {});

We mentioned the Identity Map several times already - time to test how it works. We said the entity is managed, and the Unit of Work will track its changes, and compute them when we call flush(). We also said a new entity that is marked with persist() will become managed after flushing.

Put the following code into your server.ts file, right before the orm.close() call:

// user entity is now managed, if we try to find it again, we get the same reference
const myUser = await em.findOne(User,;
console.log('users are the same?', user === myUser)

// modifying the user and flushing yields update queries = '...';
await em.flush();

Run the npm start again and verify the logs:

users are the same? true
[query] begin
[query] update `user` set `bio` = '...' where `id` = 1 [took 0 ms]
[query] commit

Next, let's try to do the same, but with an EntityManager fork:

// now try to create a new fork, does not matter if from `orm.em` or our existing `em` fork, as by default we get a clean one
const em2 = em.fork();
console.log('verify the EM ids are different:',,;
const myUser2 = await em2.findOneOrFail(User,;
console.log('users are no longer the same, as they came from different EM:', user === myUser2);

Which logs the following:

verify the EM ids are different: 3 4
[query] select `u0`.* from `user` as `u0` where `u0`.`id` = 1 limit 1 [took 0 ms]
users are no longer the same, as they came from different EM: false

We just used em.findOneOrFail() instead of em.findOne(), as you may have guessed, its purpose is to always return a value, or throw otherwise.

You can see there is a select query to load the user. This is because we used a new fork, that is clean by default - it has an empty Identity Map, and therefore it needs to load the entity from the database. In the previous example, we already had it present by the time we were calling em.findOne(). You queried the entity by its primary key, and such query will always first check the identity map and prefer the results from it instead of querying the database.

Refreshing loaded entities

The behavior described above is often what we want and serves as a first-level cache, but what if you always want to reload that entity, regardless of the existing state? There are several options:

FindOptions is the last parameter of em.find/findOne methods.

  1. fork first, to have a clear context
  2. use disableIdentityMap: true in the FindOptions
  3. use em.refresh(entity)

The first two have pretty much the same effect, using disableIdentityMap just does the forking for us behind the scenes. Let's talk about the last one - refreshing. With em.refresh(), the EntityManager will ignore the contents of the Identity Map and always fetch the entity from the database.

// change the user = 'changed';

// reload user with `em.refresh()`
await em2.refresh(myUser2);
console.log('changes are lost', myUser2);

// let's try again
myUser2!.bio = 'some change, will be saved';
await em2.flush();

Running the npm start script again, we get the following:

[query] select `u0`.* from `user` as `u0` where `u0`.`id` = 1 limit 1 [took 1 ms, 1 result]
changes are lost User {
fullName: 'Foo Bar',
email: '',
password: '123456',
bio: '...',
id: 1
[query] begin
[query] update `user` set `bio` = 'some change, will be saved' where `id` = 1 [took 0 ms, 1 row affected]
[query] commit

Removing entities

We touched on creating, reading and updating entities, the last piece of the puzzle to the CRUD riddle is the delete operation. To delete entities via EntityManager, we have two possibilities:

  1. Mark entity instance via em.remove() - this means we first need to have the entity instance. But don't worry, you can get one even without loading it from the database - via em.getReference().
  2. Fire DELETE query via em.nativeDelete() - when all you want is a simple delete query, it can be simple as that.

Let's test the first approach with removing by entity instance:

// finally, remove the entity
await em2.remove(myUser3!).flush();

Entity references

So what does the em.getReference() method mentioned above do and what is an entity reference in the first place?

MikroORM represents every entity as an object, even those that are not fully loaded. Those are called entity references - they are in fact regular entity class instances, but only with their primary key available. This makes it possible to create them without querying the database. References are stored in the identity map just like any other entity.

An alternative to the previous code snippet could be as well this:

const userRef = em.getReference(User, 1);
await em.remove(userRef).flush();

This concept is especially important for relationships and can be combined with the so-called Reference wrapper for added type safety, but we will get to that later.

Entity state and WrappedEntity

We just said that entity reference is a regular entity, but only with a primary key. How does it work? During entity discovery (which happens when you call MikroORM.init()), the ORM will patch the entity prototype and generate a lazy getter for the WrappedEntity - a class holding various metadata and state information about the entity. Each entity instance will have one, available under a hidden __helper property - to access its API in a type-safe way, use the wrap() helper:

import { wrap } from '@mikro-orm/core';

const userRef = em.getReference(User, 1);
console.log('userRef is initialized:', wrap(userRef).isInitialized());

await wrap(userRef).init();
console.log('userRef is initialized:', wrap(userRef).isInitialized());

You can also extend the BaseEntity provided by MikroORM. It defines all the public methods available via wrap() helper, so you could do userRef.isInitialized() or userRef.init().

The WrappedEntity instance also holds the state of the entity at the time it was loaded or flushed - this state is then used by the Unit of Work during flush to compute the differences. Another use case is serialization, we can use the toObject(), toPOJO() and toJSON() methods to convert the entity instance to a plain JavaScript object.

⛳ Checkpoint 1

Currently, our app consists of a single User entity and a server.ts file where we tested how to work with it using EntityManager. You can find working StackBlitz for the current state here:

Due to the nature of how the ESM support in ts-node works, it is not possible to use it inside StackBlitz project - we need to use node --loader instead. We also use in-memory database, SQLite feature available via special database name :memory:.

This is our server.ts file so far: