Pony.fm/app/Commands/AddTrackToPlaylistCommand.php

<?php

namespace Poniverse\Ponyfm\Commands;

use Poniverse\Ponyfm\Playlist;
use Poniverse\Ponyfm\Track;
use Illuminate\Support\Facades\Auth;
use Illuminate\Support\Facades\DB;

class AddTrackToPlaylistCommand extends CommandBase
{
    private $_track;
    private $_playlist;

    function __construct($playlistId, $trackId)
    {
        $this->_playlist = Playlist::find($playlistId);
        $this->_track = Track::find($trackId);
    }

    /**
     * @return bool
     */
    public function authorize()
    {
        $user = Auth::user();

        return $user != null && $this->_playlist && $this->_track && $this->_playlist->user_id == $user->id;
    }

    /**
     * @throws \Exception
     * @return CommandResponse
     */
    public function execute()
    {
        $songIndex = $this->_playlist->tracks()->count() + 1;
        $this->_playlist->tracks()->attach($this->_track, ['position' => $songIndex]);

        Playlist::whereId($this->_playlist->id)->update([
            'track_count' => DB::raw('(SELECT COUNT(id) FROM playlist_track WHERE playlist_id = ' . $this->_playlist->id . ')')
        ]);

        return CommandResponse::succeed(['message' => 'Track added!']);
    }
}