Selecting specific relationship fields

[RoyLee]

Describe the bug
Hello, I am trying to use the entity manager to query for specific fields in the relationship data. In my case I am trying to query for all of the author's books but only want the title values. Right now I am getting both the title and genre values. Can you please assist me in figuring out why this is happening?

Author.ts

@Entity({ tableName: "authors" })
export class Author extends BaseEntity {
  @Property()
  @Unique()
  name: string;

  @OneToMany('Book', 'author')
  books = new Collection<book>(this);

  constructor(name: string) {
    super();
    this.name = name;
  }
}

Book.ts

@Entity({ tableName: "books" })
export class Book extends BaseEntity {
  @Property()
  @Unique()
  title: string;

  @Property()
  genre: string;

  @ManyToOne('Author')
  author!: Author;

  constructor(title: string, genre: string) {
    super();
    this.title = title;
    this.genre = genre;
  }
}

Code that I am calling:

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

Expected behavior
I expect only the book title values and not the genre values.

Versions

| Dependency | Version |
| node | v16.13.1 |
| typescript | v4.3.5 |
| mikro-orm | v4.5.9 |
| mongodb | v4.5.9 |

[RoyLee]

One way I can think of doing this is to do 2 separate queries. One to get the author id and then another one to query books for that specific author id. This creates 2 calls to the database which is not ideal.

[RoyLee]

Hmm, looks like this is already happening when I used the query in my example. It is running 2 separate queries to the database. Can this be only one?

[RoyLee]

Just tried running:

    const books = await em().findAndCount(Book,
      { author: author.id },
      {
        fields: ['title'],
      });

Thought this returns both genre and title fields. Is my syntax missing anything?

[RoyLee]

Is this issue related to these bugs?
#1707
#1739