Sequelize selecting id instead of foreign key - mysql

Question

I have a Book table, a Category table and a join table named BookCategories. Here's an oversimplified structure:
books {
id,
name,
hash // unique
}
categories {
id,
name
}
book_categories {
id,
book_hash,
category_id
}
Because of the way the app works, it was decided to use the book_hash in the join table instead of the book_id.
Here are the Sequelize relations between the 3 tables:
// book
Book.hasMany(models.bookCategories, { foreignKey: 'book_hash' });
// category
Category.hasMany(models.bookCategories, { foreignKey: 'category_id' });
// bookCategories
BookCategories.belongsTo(models.book, { foreignKey: 'collection_hash' });
BookCategories.belongsTo(models.category, { foreignKey: 'category_id' });
I'm using Sequelize with GraphQL, so when creating the books field on the CategoryType, everything works great:
books: {
type: new GraphQLList(BookType),
args: defaultListArgs(),
async resolve(f, args) {
return await f.getBookCategories(args);
// normally doesn't return, uses results to get book hashes and return books
}
}
It runs the following queries:
SELECT `id`, `name` FROM `categories` ASC;
SELECT `id`, `book_hash`, `category_id` FROM `book_categories` AS `bookCategories` WHERE `bookCategories`.`category_id` IN (1, 2, 3, 4, 5);
But when doing it the other way around, requesting the bookCategories through the BookType, it doesn't:
categories: {
type: new GraphQLList(CategoryType),
args: defaultListArgs(),
async resolve(f, args) {
return await f.getBookCategories(args);
// normally doesn't return, uses results to get category ids and return categories
}
}
It runs these queries:
SELECT `id`, `name`, `hash`, FROM `books` AS `book` WHERE `book`.`hash` = 'ft3qvqlj';
SELECT `id`, `book_hash`, `category_id` FROM `book_categories` AS `bookCategories` WHERE `bookCategories`.`book_hash` IN (93586);
As you can see, even though I set the foreign key to the hash on both relations, it still uses the book id to do the query. Does it have to do with the fact that hash is not the primary key on the books table?
Is there a way to fix this or do I have to redo the database structure?