public static string ImportSongPerformers(MusicHubDbContext context, string xmlString)
        {
            StringBuilder stBuilder = new StringBuilder();

            var xmlSeserializer = new XmlSerializer(typeof(ImportPerformerDto[]),
                                                          new XmlRootAttribute("Performers"));
            var performersDto = (ImportPerformerDto[])xmlSeserializer.Deserialize(new StringReader(xmlString));

            var performerList = new List<Performer>();
            foreach (var performerDto in performersDto)
            {
                if (!IsValid(performerDto))
                {
                    stBuilder.AppendLine(ErrorMessage);
                    continue;
                }

                bool havesongs = true;
                var songs = new List<Song>();
                foreach (var songDto in performerDto.MappingTable)
                {
                    Song song = context.Songs.Where(s => s.Id == songDto.DtoId).FirstOrDefault();
                    if (song == null)
                    {
                        songs.Clear();
                        havesongs = false;
                        break;
                    }
                    else
                    {
                        songs.Add(song);
                    }
                }

                if (!havesongs)
                {
                    stBuilder.AppendLine(ErrorMessage);
                    continue;
                }

                var performer = new Performer
                {
                    FirstName = performerDto.FirstName,
                    LastName = performerDto.LastName,
                    Age = performerDto.Age,
                    NetWorth = performerDto.NetWorth
                };
                 context.Performers.Add(performer);

                foreach (var song in songs)
                {
                    var songPerformer = new SongPerformer
                    {
                        Song = song,
                        Performer = performer
                    };
                    context.SongsPerformers.Add(songPerformer);
                }
                // Imported { performer first name} ({ songs count}songs)
                string message = String.Format(SuccessfullyImportedPerformer, performer.FirstName, songs.Count);

                stBuilder.AppendLine(message);

            }
            string result = stBuilder.ToString().TrimEnd();
            context.SaveChanges();

            return result;
        }