Load relation ids without joining target entity

tl;dr what’s the best way to fetch relation ids from a join table?

I have a data model that is unfortunately spread out across several databases, so I need to handle some relations manually with separate queries on other connections. What I’m trying to do is find a way to fetch relation ids from a join table only; typeorm adds an additional join to the related class’s table, which in my case, exists in another database.

// Database A
@Entity('usr')
class User {
  @PrimaryGeneratedColumn()
  public id: number;

  @Column()
  public name: string;
}

// Database B
@Entity()
class Article {
  @PrimaryGeneratedColumn()
  public id: number;

  @ManyToMany(() => User)
  @JoinTable({name: 'article_authorship', inverseJoinColumn: {name: 'author_id'}})
  public authors: User[] = [];

  public authorIds: number[] = [];
}

The article_authorship join table is also on Database B and looks like:

   Column   |  Type   | Modifiers
------------+---------+-----------
 article_id | bigint  | not null
 author_id  | bigint  | not null
 ordinal    | integer | not null

I’ve tried a few different things in my controller to populate Article#authors or Article#authorIds with data, or otherwise obtain an array of ids from that join table.

SelectQueryBuilder#innerJoinAndMapMany

const articles = articleRepository.createQueryBuilder('article');

articles.innerJoinAndMapMany('article.authorIds', 'article_authorship', 'authorship', 'article.id = authorship.article_id');

This almost works. The following query is executed:

SELECT "article"."id" AS "article_id", "article"."headline" AS "article_headline", "article"."content" AS "article_content", "authorship"."article_id" AS "authorship_article_id", "authorship"."author_id" AS "authorship_author_id" FROM "article" "article" INNER JOIN "article_authorship" "authorship" ON "article"."id" = authorship.article_id WHERE ("article"."date_deleted" IS NULL) AND "article"."id" IN ($1) -- PARAMETERS: ["41255"]

For an Article with 2 authors, this does in fact load 2 records with each User id; I can see that in the raw results from SelectQueryBuilder#getRawAndEntities. However, in the single entity that’s returned, Article#authorIds is still empty. The transformation from raw results to entities leaves that out.

SelectQueryBuilder#loadAllRelationIds

const articles = articleRepository.createQueryBuilder('article');

articles.loadAllRelationIds();

This results in the following query:

SELECT "article_authors_relation_id"."author_id" AS "author_id", "article_authors_relation_id"."article_id" AS "article_id" FROM "usr" "usr" INNER JOIN "article_authorship" "article_authors_relation_id" ON ("article_authors_relation_id"."article_id" = $1 AND "article_authors_relation_id"."author_id" = "usr"."id") ORDER BY "article_authors_relation_id"."author_id" ASC, "article_authors_relation_id"."article_id" ASC -- PARAMETERS: ["41255"]

Obviously, this fails outright, as the usr table is on Database A. I also attempted to use SelectQueryBuilder#loadRelationIdAndMap) and heavily modify the query but no luck there.