用ActiveRecord实现三个表之间的Yii2关系

// User表
class User extends \yii\db\ActiveRecord
{
    public static function tableName()
    {
        return 'user';
    }

    public function getPosts()
    {
        return $this->hasMany(Post::className(), ['user_id' => 'id']);
    }
}

// Post表
class Post extends \yii\db\ActiveRecord
{
    public static function tableName()
    {
        return 'post';
    }

    public function getUser()
    {
        return $this->hasOne(User::className(), ['id' => 'user_id']);
    }

    public function getComments()
    {
        return $this->hasMany(Comment::className(), ['post_id' => 'id']);
    }
}

// Comment表
class Comment extends \yii\db\ActiveRecord
{
    public static function tableName()
    {
        return 'comment';
    }

    public function getPost()
    {
        return $this->hasOne(Post::className(), ['id' => 'post_id']);
    }
}

$user = User::findOne($userId);
$posts = $user->posts;

$post = Post::findOne($postId);
$comments = $post->comments;