java - object references an unsaved transient instance error -

ok want mapped groups , student in 1 table i've got error: "object references unsaved transient instance". if add cascade objects arleady exists. wrong ?

here entities:

 @entity    @table(name = "listaosob")   public class listaosob implements serializable {  @id @generatedvalue private int id; private static final long serialversionuid = 1l; @manytoone private student student; @manytoone private grupa grupa;  @entity   @table(name = "student")  public class student implements serializable {  private static final long serialversionuid = 1l; @id @generatedvalue private int id; private string imie; private string nazwisko; private string login; private string haslo; @column(unique=true) private int nr_albumu; @column(unique=true) private string kod_karty;   @entity    @table(name = "grupa")   public class grupa implements serializable {  private static final long serialversionuid = 1l; @id @generatedvalue private int id; private string nazwa; private time godzina; private string sala; private string tydzien; @manytoone private prowadzacy prowadzacy; 

ejb code:

 @stateless  @localbean  public class studentbean implements studentbeanremote, studentbeanlocal {   @persistencecontext    private entitymanager em;   public studentbean() {}  public void dodajstudenta(student s) {     em.persist(s);     em.flush(); }  public void usunstudenta(student s) {     em.remove(s);       em.flush(); }  @suppresswarnings("unchecked") public arraylist<student> listastudentow() {     query query = em.createquery("select e student e");     return (arraylist<student>) query.getresultlist(); }  @suppresswarnings("unchecked") public arraylist<student> listastudentow(grupa grupa) {     query query = em.createquery("select e listaosob e e.grupa = :grupa");     query.setparameter("grupa", grupa);     return (arraylist<student>) query.getresultlist(); }  public void dodajgrupe(grupa g) {     em.persist(g); }  public void usungrupe(grupa g) {     em.remove(g);       em.flush(); }  public void dodajlisteosob(listaosob l) {     em.persist(l); } 

and example test:

student name1 = new student("new"+gen.nextint(),"nazwisko","log", "haslo", "asd"+gen.nextint(), gen.nextint());             obj.dodajstudenta(name1);             grupa n = new grupa();             obj.dodajgrupe(n);             listaosob lista = new listaosob(name1, n);             obj.dodajlisteosob(lista); 

dodajlisteosob throws object references unsaved transient instance

• because listaosob contains grupa (@manytoone), not present in database
• grupa not present in database, because dodajgrupe not persist other methods do
• also dodajlisteosob not call persist (in scenario not cause issues)