class Catalogue {

List books
String book
static hasMany = [books: book]

}

class Catalogue {

List books

}

class Catalogue {

String[] books

}

class Catalogue {

ArrayList<String> books = new ArrayList<String>()

}

class Catalogue {
    static hasMany = [books: String]
    List books
}

class Book {
 String title
 String isbn
 ...

String toString(){
  return "${title}"
 }

class Catalogue{
  String name
  ....
  static hasMany = [books: Book]
  //etc etc
}