laravel-mongodb : How to add a foreign key to two existing collections?

Solution:

Try like this:

$last_new_id = 1;

foreach($categories as $category){
    $category->id = $last_new_id ;
    $last_new_id++;
}

foreach($books as $book){
    $book->category_id = $categories->where('category',$book->category)->first()->id
}