/**
* Tidy up the library by deleting empty albums and artists.
*/
public function tidy()
{
$inUseAlbums = Song::select('album_id')->groupBy('album_id')->get()->pluck('album_id')->toArray();
$inUseAlbums[] = Album::UNKNOWN_ID;
Album::deleteWhereIDsNotIn($inUseAlbums);
$inUseArtists = Album::select('artist_id')->groupBy('artist_id')->get()->pluck('artist_id')->toArray();
$contributingArtists = Song::distinct()->select('contributing_artist_id')->groupBy('contributing_artist_id')->get()->pluck('contributing_artist_id')->toArray();
$inUseArtists = array_merge($inUseArtists, $contributingArtists);
$inUseArtists[] = Artist::UNKNOWN_ID;
$inUseArtists[] = Artist::VARIOUS_ID;
Artist::deleteWhereIDsNotIn(array_filter($inUseArtists));
}