Finding all specific relations based on another pivot relation - laravel

Question

I know the title of my question is really weird! I hope I can clarify it with explanations. I have three models, Book, Auther, and AutherType. My tables structure :
table books:
id
title
1
book1
2
book2
table authors:
id
name
1
Bob
2
Sara
table author_types:
id
title
1
writer
2
translator
Each Book can have multiple Authors with a type. The pivot table of this relationship is something like that:
book_id
author_id
author_type_id
1
1
1
1
2
2
2
2
1
Also, I have authors(general relation) and writers(partial relation) relationship in my Book model:
public function authors()
{
return $this->belongsToMany('App\Models\Author', 'author_book', 'book_id')->withPivot('author_type_id');
}
public function writers()
{
return $this->belongsToMany('App\Models\Author', 'author_book', 'book_id')
->wherHas('authorType', function($q){
$q->where('title', 'writer');
})
->withPivot('author_type_id');
}
My question is that, how can I fetch all specific type of a book? For example, all writers of a book. In this case, the writer of the book1 is Bob. But my writers relation cannot satisfy this. The output of $book->writers for the book1 contains Bob and Sara. Because Sara has writer role for another book!

Answer 1

While there is no ability to write queries on complex pivots (belongsToMany relations), we have two choices:
First(which is the simplest one): get the writer type id and pass it through the relation, like:
public function writers()
{
$writer_id = AuthorType::where('title', 'writer')->first()->id;
return $this->authors()->wherePivot('author_type_id', $writer_id);
}
Second (The heavy choice), write a new method in Author model that gives the author types for a specific book and then in Book model filter authors according there types for this book:
in Author.php:
public function authorTypes()
{
return $this->belongsToMany('App\Models\AuthorType', 'author_book');
}
public function authorTypesForBook($book_id){
return $this->authorTypes()->wherePivot('book_id', '=' , $book_id);
}
and in Book.php:
public function writers()
{
$authors = $this->belongsToMany('App\Models\Author', 'author_book')->withPivot('author_type_id')->get();
$book_id = $this->id;
return $authors->filter(function ($item) use ($book_id){
return $item->authorTypesForBook($book_id)->get()->contains(function($value){
return $value->title == 'writer';
});
});
}
Hope this would help.