MikroORM: How to set foreign key by raw id?

In MikroORM, how can you set a foreign key by its raw numeric value? For example, in Django ORM you can do this:

b = new Book()
b.author = someAuthorWithId1

and this, with raw id

b = new Book()
b.author_id = 1

How can use raw id in MikroORM?

2024 update

Nowadays (v6+) there is also the rel and ref helpers that can create unmanaged entity reference:

const b = new Book();
// or ref() if the relation uses Reference wrapper
b.author = rel(Author, 1);

https://mikro-orm.io/docs/entity-constructors#pojo-vs-entity-instance-in-constructor

There are several ways:

1. using references:

const b = new Book();
b.author = em.getReference(Author, 1);

2. using assign helper:

const b = new Book();
em.assign(b, { author: 1 });

3. using create helper:

const b = em.create(Book, { author: 1 });