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Version: 4.4

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, 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', '');
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

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

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#

This feature is supported only for SELECT_IN loading strategy.

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

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

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

Or with an alternative object syntax:

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

It is also possible to use multiple levels:

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

Primary keys are always selected even if you omit them. On the other hand, you are responsible for selecting the FKs - if you 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 orm.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 you need to make sure to mark such properties too.

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

Using custom SQL fragments#

It is possible to use any SQL fragment in your 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 orm.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, loclongitude) <@> 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, you first need to merge them via em.registerManaged().

const users = await orm.em.find(User, { email: 'foo@bar.baz' }, {
disableIdentityMap: true,
populate: { cars: { brand: true } },
users[0].name = 'changed';
await orm.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 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.

Last updated on by Martin Adámek