laravel 5.4多态关系-通过查询作用域进行急切读取

// 定义多态关系
class Comment extends Model
{
    public function commentable()
    {
        return $this->morphTo();
    }
}

class Post extends Model
{
    public function comments()
    {
        return $this->morphMany(Comment::class, 'commentable');
    }
}

class Video extends Model
{
    public function comments()
    {
        return $this->morphMany(Comment::class, 'commentable');
    }
}

// 定义查询作用域
class CommentScope
{
    public function scopeWithRelations($query)
    {
        return $query->with('commentable');
    }
}

// 使用查询作用域进行急切读取
$comments = Comment::with(['commentable' => function ($query) {
    $query->withRelations();
}])->get();

foreach ($comments as $comment) {
    echo $comment->commentable->title;
}