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Version: 5.4

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) 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.

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, we do not have to call persist on those, and flush is enough to update them.

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

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

Persisting and Cascading

To save entity state to database, we 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 our 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 em.persistAndFlush([book1, book2, book3]);

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

Fetching Entities with EntityManager

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


const author = await em.findOne(Author, '');
const books = await 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

To populate entity relations, we can use populate parameter.

const books = await em.find(Book, { foo: 1 }, { populate: ['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 em.createQueryBuilder(Author).select('*').getResult();
await em.populate(authors, { populate: ['books.tags'] });

// now our 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 em.find(User, { firstName: 'John' });

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

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

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

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

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

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

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

Using custom classes in FilterQuery

If we decide to abstract the filter options in our own object then we might run into the problem that the find option does not return the results we'd expect. This is due to the fact that the FilterQuery should be provided as a plain object (POJO), and not a class instance with prototype.

If we want to provide our own FilterQuery DTO, then our DTO class should extend the PlainObject class. This way MikroORM knows it should be treated as such.

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

class Filter extends PlainObject {
name: string;

const where = new Filter(); = 'Jon';
const res = await em.find(Author, where);

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

Sometimes we 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, we can always type cast the query to any.

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

Another problem we 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 we 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 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 em.findOne(Author, { books: { tags: { name: 'Tag name' } } }, { populate: ['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 we 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

This feature is supported only for SELECT_IN loading strategy.

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

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

From v4.4 it is also possible to specify fields for nested relations:

const author = await em.findOne(Author, '...', { fields: ['name', 'books.title', '', 'books.price'] });

Or with an alternative object syntax:

const author = await em.findOne(Author, '...', { fields: ['name', { books: ['title', 'author', 'price'] }] });

It is also possible to use multiple levels:

const author = await em.findOne(Author, '...', { fields: ['name', { books: ['title', 'price', 'author', { author: ['email'] }] }] });

Primary keys are always selected even if we omit them. On the other hand, we are responsible for selecting the FKs - if we omit such property, the relation might not be loaded properly. In the following example the books would not be linked the author, because we did not specify the field to be loaded.

// this will load both author and book entities, but they won't be connected due to the missing FK in select
const author = await em.findOne(Author, '...', { fields: ['name', { books: ['title', 'price'] });

Same problem can occur in mongo with M:N collections - those are stored as array property on the owning entity, so we need to make sure to mark such properties too.

Fetching Paginated Results

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

const [authors, count] = await 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 we call em.findOne() and no entity is found based on our criteria, null will be returned. If we rather have an Error instance thrown, we can use em.findOneOrFail():

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

try {
const author = await 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 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); // our custom error

Using custom SQL fragments

It is possible to use any SQL fragment in our WHERE query or ORDER BY clause:

The expr() helper is an identity function - all it does is to return its parameter. We can use it to bypass the strict type checks in FilterQuery.

const users = await em.find(User, { [expr('lower(email)')]: 'foo@bar.baz' }, {
orderBy: { [`(point(loc_latitude, loc_longitude) <@> point(0, 0))`]: 'ASC' },

This will produce following query:

select `e0`.*
from `user` as `e0`
where lower(email) = 'foo@bar.baz'
order by (point(loc_latitude, loc_longitude) <@> point(0, 0)) asc

Disabling identity map and change set tracking

Sometimes we might want to disable identity map and change set tracking for some query. This is possible via disableIdentityMap option. Behind the scenes, it will create new context, load the entities inside that, and clear it afterwards, so the main identity map will stay clean.

As opposed to managed entities, such entities are called detached. To be able to work with them, we first need to merge them via em.registerManaged().

const users = await em.find(User, { email: 'foo@bar.baz' }, {
disableIdentityMap: true,
populate: ['cars.brand'],
users[0].name = 'changed';
await em.flush(); // calling flush have no effect, as the entity is not managed

Keep in mind that this can also have negative effect on the performance.

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

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

Entity Repositories

Although we can use EntityManager directly, much more convenient way is to use EntityRepository instead. You can register our repositories in dependency injection container like InversifyJS so we 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.

Custom Property Ordering

Entity properties provide some support for custom ordering via the customOrder attribute. This is useful for values that have a natural order that doesn't align with their underlying data representation. Consider the code below, the natural sorting order would be high, low, medium. However we can provide the customOrder to indicate how the enum values should be sorted.

enum Priority { Low = 'low', Medium = 'medium', High = 'high' }
class Task {
id!: number

label!: string

items: () => Priority,
customOrder: [Priority.Low, Priority.Medium, Priority.High]
priority!: Priority

// ...

await em.persistAndFlush([
em.create(Task, { label: 'A', priority: Priority.Low }),
em.create(Task, { label: 'B', priority: Priority.Medium }),
em.create(Task, { label: 'C', priority: Priority.High })

const tasks = await em.find(Task, {}, { orderBy: { priority: QueryOrder.ASC } });
for (const t of tasks) {
// Logs A, B, C