public ConcertPerson(Concert concert, Person person, int rank = 1)
 {
     ConcertId = concert.Id;
     PersonId  = person.Id;
     Rank      = rank;
 }
 public ConcertArtist(Concert concert, Artist artist, int rank = 1)
 {
     ConcertId = concert.Id;
     ArtistId  = artist.Id;
     Rank      = rank;
 }