Eloquent: Use another column as primary key on a relationship

Since at least 4.2, you can now specify the local key in the hasMany.

From the documentation (4.2, 5.2, 5.4, 5.7) :

return $this->hasMany('Comment', 'foreign_key', 'local_key');

Stumbled into the same problem lately, I'm not yet sure whether this would work, but as an idea:

class Book {
    public function isbn() {
        // instead of hasMany
        return ISBN::where('isbn', $this->isbn);
    }
}