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Version: 3.6


OneToMany and ManyToMany collections are stored in a Collection wrapper. It implements iterator so you can use for of loop to iterate through it.

Another way to access collection items is to use bracket syntax like when you access array items. Keep in mind that this approach will not check if the collection is initialed, while using get method will throw error in this case.

Note that array access in Collection is available only for reading already loaded items, you cannot add new items to Collection this way.

const author = orm.em.findOne(Author, '...', ['books']); // populating books collection

// or we could lazy load books collection later via `init()` method
await author.books.init();

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

// collection needs to be initialized before you can work with it
console.log(author.books.contains(book)); // true
console.log(author.books.contains(book)); // false
console.log(author.books.count()); // 1
console.log(author.books.contains(book)); // false
console.log(author.books.count()); // 0
console.log(author.books.getItems()); // Book[]
console.log(author.books.getIdentifiers()); // array of string | number
console.log(author.books.getIdentifiers('_id')); // array of ObjectId

// array access works as well
console.log(author.books[1]); // Book
console.log(author.books[12345]); // undefined, even if the collection is not initialized

const author = orm.em.findOne(Author, '...'); // books collection has not been populated
console.log(author.books.getItems()); // throws because the collection has not been initialized
// initialize collection if not already loaded and return its items as array
console.log(await author.books.loadItems()); // Book[]

OneToMany Collections

OneToMany collections are inverse side of ManyToOne references, to which they need to point via fk attribute:

export class Book {

_id!: ObjectId;

author!: Author;


export class Author {

_id!: ObjectId;

@OneToMany(() => Book, book =>
books1 = new Collection<Book>(this);

// or via options object
@OneToMany({ entity: () => Book, mappedBy: 'author' })
books2 = new Collection<Book>(this);


ManyToMany Collections

For ManyToMany, SQL drivers use pivot table that holds reference to both entities.

As opposed to them, with MongoDB we do not need to have join tables for ManyToMany relations. All references are stored as an array of ObjectIds on owning entity.


Unidirectional ManyToMany relations are defined only on one side, if you define only entity attribute, then it will be considered the owning side:

@ManyToMany(() => Book)
books1 = new Collection<Book>(this);

// or mark it as owner explicitly via options object
@ManyToMany({ entity: () => Book, owner: true })
books2 = new Collection<Book>(this);


Bidirectional ManyToMany relations are defined on both sides, while one is owning side (where references are store), marked by inversedBy attribute pointing to the inverse side:

@ManyToMany(() => BookTag, tag => tag.books, { owner: true })
tags = new Collection<BookTag>(this);

// or via options object
@ManyToMany({ entity: () => BookTag, inversedBy: 'books' })
tags = new Collection<BookTag>(this);

And on the inversed side we define it with mappedBy attribute pointing back to the owner:

@ManyToMany(() => Book, book => book.tags)
books = new Collection<Book>(this);

// or via options object
@ManyToMany({ entity: () => Book, mappedBy: 'tags' })
books = new Collection<Book>(this);

Forcing fixed order of collection items

Since v3 many to many collections does not require having auto increment primary key, that was used to ensure fixed order of collection items.

To preserve fixed order of collections, you can use fixedOrder: true attribute, which will start ordering by id column. Schema generator will convert the pivot table to have auto increment primary key id. You can also change the order column name via fixedOrderColumn: 'order'.

You can also specify default ordering via orderBy: { ... } attribute. This will be used when you fully populate the collection including its items, as it orders by the referenced entity properties instead of pivot table columns (which fixedOrderColumn is). On the other hand, fixedOrder is used to maintain the insert order of items instead of ordering by some property.

Propagation of Collection's add() and remove() operations

When you use one of Collection.add() method, the item is added to given collection, and this action is also propagated to its counterpart.

// one to many
const author = new Author(...);
const book = new Book(...);

console.log(; // author will be set thanks to the propagation

For M:N this works in both ways, either from owning side, or from inverse side.

// many to many works both from owning side and from inverse side
const book = new Book(...);
const tag = new BookTag(...);

console.log(tag.books.contains(book)); // true

console.log(book.tags.contains(tag)); // true

Collections on both sides have to be initialized, otherwise propagation won't work.

Although this propagation works also for M:N inverse side, you should always use owning side to manipulate the collection.

Same applies for Collection.remove().

Filtering and ordering of collection items

When initializing collection items via collection.init(), you can filter the collection as well as order its items:

await book.tags.init({ where: { active: true }, orderBy: { name: QueryOrder.DESC } });

You should never modify partially loaded collection.