Laravel Many To Many Save

I have a tag and post models.

App\Tag

public function posts()
    {
        return $this->belongsToMany('App\Post','post_tag','post_id');
    }

App\Post

    public function tags()
        {
            return $this->belongsToMany('App\Tag','post_tag','tag_id')->withTimestamps(); 
}

I want to add tag when add post.

PostController@store

public function store(Request $request)
    {
        \Auth::user()->posts()->save(new Post($request->all()));
        return \Redirect::route('posts.index');
    }

How do I use this save method in my store function?

App\User::find(1)->roles()->save($role, ['expires' => $expires]);

via Chebli Mohamed