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.

const book = await orm.em.findOne(Book, 1);
book.title = 'How to persist things...';

// no need to persist `book` as its already managed by the EM
await orm.em.flush();

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', 'snow@wall.st');
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
orm.em.persistLater(book1);
orm.em.persistLater(book2);
orm.em.persistLater(book3);
await orm.em.flush(); // flush everything to database at once

Auto-flushing

Since MikroORM v3, default value for autoFlush is false. That means you need to call em.flush() yourself to persist changes into database. You can still change this via ORM's options to ease the transition but generally it is not recommended as it can cause unwanted small transactions being created around each persist.

orm.em.persist(new Entity()); // no auto-flushing by default
await orm.em.flush();
await orm.em.persist(new Entity(), true); // you can still use second parameter to auto-flush

Fetching Entities with EntityManager

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

Example:

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

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

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

To populate entity relations, you can use populate parameter.

const books = await orm.em.find(Book, { foo: 1 }, ['author.friends']);

You can also use em.populate() helper to populate relations (or to ensure they are fully populated) on already loaded entities. This is also handy when loading entities via QueryBuilder:

const authors = await orm.em.createQueryBuilder(Author).select('*').getResult();
await em.populate(authors, ['books.tags']);

// now your Author entities will have `books` collections populated,
// as well as they will have their `tags` collections populated.
console.log(authors[0].books[0].tags[0]); // initialized BookTag

Conditions Object (FilterQuery<T>)

Querying entities via conditions object (where in em.find(Entity, where: FilterQuery<T>)) supports many different ways:

// search by entity properties
const users = await orm.em.find(User, { firstName: 'John' });

// for searching by reference you can use primary key directly
const id = 1;
const users = await orm.em.find(User, { organization: id });

// or pass unpopulated reference (including `Reference` wrapper)
const ref = await orm.em.getReference(Organization, id);
const users = await orm.em.find(User, { organization: ref });

// fully populated entities as also supported
const ent = await orm.em.findOne(Organization, id);
const users = await orm.em.find(User, { organization: ent });

// complex queries with operators
const users = await orm.em.find(User, { $and: [{ id: { $nin: [3, 4] } }, { id: { $gt: 2 } }] });

// you can also search for array of primary keys directly
const users = await orm.em.find(User, [1, 2, 3, 4, 5]);

// and in findOne all of this works, plus you can search by single primary key
const user1 = await orm.em.findOne(User, 1);

As you can see in the fifth example, one can also use operators like $and, $or, $gte, $gt, $lte, $lt, $in, $nin, $eq, $ne, $like, $re. More about that can be found in Query Conditions section.

Mitigating Type instantiation is excessively deep and possibly infinite.ts(2589) error

Sometimes you might be facing TypeScript errors caused by too complex query for it to properly infer all types. Usually it can be solved by providing the type argument explicitly.

You can also opt in to use repository instead, as there the type inference should not be problematic.

As a last resort, you can always type cast the query to any.

const books = await orm.em.find<Book>(Book, { ... your complex query ... });
// or
const books = await orm.em.getRepository(Book).find({ ... your complex query ... });
// or
const books = await orm.em.find<any>(Book, { ... your complex query ... }) as Book[];

Another problem you might be facing is RangeError: Maximum call stack size exceeded error thrown during TypeScript compilation (usually from file node_modules/typescript/lib/typescript.js). The solution to this is the same, just provide the type argument explicitly.

Searching by referenced entity fields

You can also search by referenced entity properties. Simply pass nested where condition like this and all requested relationships will be automatically joined. Currently it will only join them so you can search and sort by those. To populate entities, do not forget to pass the populate parameter as well.

// find author of a book that has tag specified by name
const author = await orm.em.findOne(Author, { books: { tags: { name: 'Tag name' } } });
console.log(author.books.isInitialized()); // false, as it only works for query and sort

const author = await orm.em.findOne(Author, { books: { tags: { name: 'Tag name' } } }, ['books.tags']);
console.log(author.books.isInitialized()); // true, because it was populated
console.log(author.books[0].tags.isInitialized()); // true, because it was populated
console.log(author.books[0].tags[0].isInitialized()); // true, because it was populated

This feature is fully available only for SQL drivers. In MongoDB always you need to query from the owning side - so in the example above, first load book tag by name, then associated book, then the author. Another option is to denormalize the schema.

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(author.id); // PK is always selected
console.log(author.name); // Jon Snow
console.log(author.email); // undefined

Fetching Paginated Results

If you are going to paginate your results, you can use em.findAndCount() that will return total count of entities before applying limit and offset.

const [authors, count] = await orm.em.findAndCount(Author, { ... }, { limit: 10, offset: 50 });
console.log(authors.length); // based on limit parameter, e.g. 10
console.log(count); // total count, e.g. 1327

Handling Not Found Entities

When you call em.findOne() and no entity is found based on your criteria, null will be returned. If you rather have an Error instance thrown, you can use em.findOneOrFail():

const author = await orm.em.findOne(Author, { name: 'does-not-exist' });
console.log(author === null); // true

try {
  const author = await orm.em.findOneOrFail(Author, { name: 'does-not-exist' });
  // author will be always found here
} catch (e) {
  console.error('Not found', e);
}

You can customize the error either globally via findOneOrFailHandler option, or locally via failHandler option in findOneOrFail call.

try {
  const author = await orm.em.findOneOrFail(Author, { name: 'does-not-exist' }, {
    failHandler: (entityName: string, where: Record<string, any> | IPrimaryKey) => new Error(`Failed: ${entityName} in ${util.inspect(where)}`)
  });
} catch (e) {
  console.error(e); // your custom error
}

Type of Fetched Entities

Both em.find and em.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>(Author.name, '...id...');
const author2 = await orm.em.findOne<Author>('Author', '...id...');
const author3 = await orm.em.findOne(Author, '...id...');

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 AnyEntity>(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 AnyEntity>(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;
  schema?: string;
}

---

find<T extends AnyEntity>(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.

---

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

Combination of find and count methods.

---

findOne<T extends AnyEntity>(entityName: string | EntityClass<T>, where: FilterQuery<T> | IPrimaryKey, 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.

---

findOneOrFail<T extends AnyEntity>(entityName: string | EntityClass<T>, where: FilterQuery<T> | IPrimaryKey, populate?: string[]): Promise<T>

Just like findOne, but throws when entity not found, so it always resolves to given entity. You can customize the error either globally via findOneOrFailHandler option, or locally via failHandler option in findOneOrFail call.

---

merge<T extends AnyEntity>(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 AnyEntity>(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 em.merge().

---

getReference<T extends AnyEntity>(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?: FilterQuery<T>): Promise<number>

Gets count of entities matching the where condition.

---

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

Shortcut for persist & flush.

---

persistLater(entity: AnyEntity | AnyEntity[]): 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: AnyEntity | FilterQuery<T> | IPrimaryKey, 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: AnyEntity, 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: AnyEntity): Promise<void>

Shortcut for removeEntity & flush.

---

removeLater(entity: AnyEntity): 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).

---