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Version: 2.7

Working with Entity Manager

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, flush?: boolean) is used to mark new entities for future persisting. It will make the entity managed by given EntityManager and once flush will be called, it will be written to the database. Second boolean parameter can be used to invoke flush immediately. Its default value is configurable via autoFlush option.

To understand flush, lets first define what managed entity is: An entity is managed if it’s fetched from the database (via em.find(), em.findOne() or via other managed entity) or registered as new through em.persist().

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

Persisting and cascading

To save entity state to database, you need to persist it. Persist determines whether to use insert or update and computes appropriate change-set. Entity references that are not persisted yet (does not have identifier) will be cascade persisted automatically.

// use constructors in your entities for required parameters
const author = new Author('Jon Snow', '');
author.born = new Date();

const publisher = new Publisher('7K publisher');

const book1 = new Book('My Life on The Wall, part 1', author);
book1.publisher = publisher;
const book2 = new Book('My Life on The Wall, part 2', author);
book2.publisher = publisher;
const book3 = new Book('My Life on The Wall, part 3', author);
book3.publisher = publisher;

// just persist books, author and publisher will be automatically cascade persisted
await orm.em.persistAndFlush([book1, book2, book3]);

// or one by one
await orm.em.flush(); // flush everything to database at once

Auto flushing

By default, EntityManager.persist() will flush your changes automatically. You can use its second parameter to disable auto-flushing, and use EntityManager.flush() manually.

You can also disable this feature globally via autoFlush option when initializing the ORM:

const orm = await MikroORM.init({
autoFlush: false,
// ...
await orm.em.persist(new Entity()); // no auto-flushing now
await orm.em.flush();
await orm.em.persist(new Entity(), true); // you can still use second parameter to auto-flush

Default value of autoFlush is currently set to true, which will change in upcoming major release. Users are encouraged to either set autoFlush to false or use em.persistLater() (equal to em.persist(entity, false)) and em.persistAndFlush() methods instead.

Fetching entities with EntityManager

To fetch entities from database you can use find() and findOne() of EntityManager:


const author = await orm.em.findOne(Author, '');
const books = await orm.em.find(Book, {});

for (const author of authors) {
console.log(; // Jon Snow

for (const book of author.books) {
console.log(book.title); // initialized
console.log(; // true
console.log(; // Jon Snow
console.log(book.publisher); // just reference
console.log(book.publisher.isInitialized()); // false
console.log(; // undefined

Fetching partial entities

When fetching single entity, you can choose to select only parts of an entity via options.fields:

const author = await orm.em.findOne(Author, '...', { fields: ['name', 'born'] });
console.log(; // PK is always selected
console.log(; // Jon Snow
console.log(; // undefined

Type of fetched entities

Both EntityManager.find and EntityManager.findOne() methods have generic return types. All of following examples are equal and will let typescript correctly infer the entity type:

const author1 = await orm.em.findOne<Author>(, '');
const author2 = await orm.em.findOne<Author>('Author', '');
const author3 = await orm.em.findOne(Author, '');

As the last one is the least verbose, it should be preferred.

Entity repositories

Although you can use EntityManager directly, much more convenient way is to use EntityRepository instead. You can register your repositories in dependency injection container like InversifyJS so you do not need to get them from EntityManager each time.

For more examples, take a look at tests/EntityManager.mongo.test.ts or tests/EntityManager.mysql.test.ts.

EntityManager API

getRepository<T extends IEntity>(entityName: string | EntityClass<T>): EntityRepository<T>

Returns EntityRepository for given entity, respects customRepository option of @Entity and entityRepository option of MikroORM.init().

find<T extends IEntity>(entityName: string | EntityClass<T>, where?: FilterQuery<T>, options?: FindOptions): Promise<T[]>

Returns array of entities found for given condition. You can specify FindOptions to request population of referenced entities or control the pagination:

export interface FindOptions {
populate?: string[];
orderBy?: { [k: string]: QueryOrder };
limit?: number;
offset?: number;

find<T extends IEntity>(entityName: string | EntityClass<T>, where?: FilterQuery<T>, populate?: string[], orderBy?: { [k: string]: QueryOrder }, limit?: number, offset?: number): Promise<T[]>

Same as previous find method, just with dedicated parameters for populate, orderBy, limit and offset.

findOne<T extends IEntity>(entityName: string | EntityClass<T>, where: FilterQuery<T> | string, populate?: string[]): Promise<T | null>

Finds an entity by given where condition. You can use primary key as where value, then if the entity is already managed, no database call will be made.

merge<T extends IEntity>(entityName: string | EntityClass<T>, data: EntityData<T>): T

Adds given entity to current Identity Map. After merging, entity becomes managed. This is useful when you want to work with cached entities.

map<T extends IEntity>(entityName: string | EntityClass<T>, data: EntityData<T>): T

Maps raw DB result to entity, adding it to current Identity Map. Equivalent to IDatabaseDriver.mapResult() followed by EntityManager.merge().

getReference<T extends IEntity>(entityName: string | EntityClass<T>, id: string): T

Gets a reference to the entity identified by the given type and identifier without actually loading it, if the entity is not yet loaded.

count(entityName: string | EntityClass<T>, where: any): Promise<number>

Gets count of entities matching the where condition.

persist(entity: IEntity | IEntity[], flush?: boolean): Promise<void>

Tells the EntityManager to make an instance managed and persistent. The entity will be entered into the database at or before transaction commit or as a result of the flush operation. You can control immediate flushing via flush parameter and via autoFlush configuration option.

persistAndFlush(entity: IEntity | IEntity[]): Promise<void>

Shortcut for persist & flush.

persistLater(entity: IEntity | IEntity[]): void

Shortcut for just persist, without flushing.

flush(): Promise<void>

Flushes all changes to objects that have been queued up to now to the database.

remove(entityName: string | EntityClass<T>, where: IEntity | any, flush?: boolean): Promise<number>

When provided entity instance as where value, then it calls removeEntity(entity, flush), otherwise it fires delete query with given where condition.

This method fires beforeDelete and afterDelete hooks only if you provide entity instance.

removeEntity(entity: IEntity, flush?: boolean): Promise<number>

Removes an entity instance. A removed entity will be removed from the database at or before transaction commit or as a result of the flush operation. You can control immediate flushing via flush parameter and via autoFlush configuration option.

This method fires beforeDelete and afterDelete hooks.

removeAndFlush(entity: IEntity): Promise<void>

Shortcut for removeEntity & flush.

removeLater(entity: IEntity): void

Shortcut for removeEntity without flushing.

clear(): void

Clears the EntityManager. All entities that are currently managed by this EntityManager become detached.

canPopulate(entityName: string | EntityClass<T>, property: string): boolean

Returns whether given entity has given property which can be populated (is reference or collection).