trigger UpdateTrackCount on Track__c (before insert, after update,  after delete, after undelete) {
    if ( Trigger.isUndelete) {
        Set <Song__c> songsToBeUpdated = new Set <Song__c> ();
        Set <Id> songId = new Set <Id>();
        for (Track__c track : Trigger.New) {
            songId.add(track.Song__c);
        }
        System.debug(songId);
        Map <Id, Song__c> usedSong = new Map  <Id, Song__c> ([SELECT id, Track_Count__c, Track_Licenses__c FROM Song__c  WHERE Id in :songId]);
        System.debug(usedSong);
        for (Track__c track : Trigger.New){
            Song__c currentSong =  usedSong.get(track.Song__c);
            if (currentSong.Track_Count__c == currentSong.Track_Licenses__c){
                track.addError('You exeeded the limits of using that song.');
            }
            else{
                currentSong.Track_Count__c++;
            }
        }
        	update usedSong.values();

    }
    if(Trigger.isDelete){
        List <Song__c> songs = new List <Song__c> ();
        List <Id> songId = new List <Id>();
        for (Track__c track : Trigger.Old) {
            songId.add(track.Song__c);
        }
        for (Song__c song : [Select id,Track_Count__c,Track_Licenses__c FROM Song__c WHERE Id in :songId]) {
            song.Track_Count__c--;
            songs.add(song);
        }
        update songs;
    }
   if(Trigger.isUndelete){

    }
   if(Trigger.isUpdate){
    }
}