PHP活动记录更新关系表

Question

这是我的多对多关系

class Post extends ActiveRecord\Model {
    static $table_name = 'app_post';

    static $has_many = array(
        array('postcategory'),
        array('category', 'through' => 'postcategory')
    );
}

class Category extends ActiveRecord\Model {
    static $table_name = 'app_post_category';

    static $has_many = array(
        array('post', 'through' => 'postcategory'),
        array('postcategory')
    );
}

class PostCategory extends ActiveRecord\Model {
    static $table_name = 'app_post_category_relation';

    static $belongs_to = array(
        array('post'),
        array('category')
    );
}

要在帖子中创建新记录，请使用

$post = Post::create(
    array(
        'title' => 'Title', 
        'description' => 'Description', 
        'keyword' => 'key,word'
    )
);

$post->create_postcategory(array('category_id' => '1'));
$post->create_postcategory(array('category_id' => '3'));

但我不知道如何更新此记录？

$post = Post::find(1);
$post->title = 'Some title';
$post->save();

这只是更新帖子表关系表怎么样？