以下是一个示例，展示了如何在 Backpack for Laravel 中定义相关的模型关系，并使用中间表进行多对多关联，并在删除主模型时自动删除中间表中对应的条目。

class Author extends Model
{
    public function books()
    {
        return $this->belongsToMany(Book::class)->withTimestamps();
    }
}

class Book extends Model
{
    public function authors()
    {
        return $this->belongsToMany(Author::class)->withTimestamps();
    }
}

// 中间表 authors_books
Schema::create('authors_books', function (Blueprint $table) {
    $table->integer('author_id')->unsigned();
    $table->foreign('author_id')->references('id')->on('authors')->onDelete('cascade');

    $table->integer('book_id')->unsigned();
    $table->foreign('book_id')->references('id')->on('books')->onDelete('cascade');

    $table->timestamps();
});

// 在 Author 和 Book 模型中添加以下代码，以在删除主模型时自动删除相关中间表条目
class Author extends Model
{
    public static function boot()
    {
        parent::boot();

        static::deleting(function ($author) {
            $author->books()->detach();
        });
    }
}

class Book extends Model
{
    public static function boot()
    {
        parent::boot();

        static::deleting(function ($book) {
            $book->authors()->detach();
        });
    }
}