ZK + hibernate: OneToMany and ManyToOne didn't work

@Entity
@Table(name = "server")
public class Server
{
   ....
  @OneToMany(mappedBy="server", fetch=FetchType.EAGER)
  @Column(name="id")
  protected Set<Site> sites;
  ....
  public void addSite(Site site)
  {
    site.setServer(this);
    this.getSites().add(site);
  }
  
  public void removeSite(Site site)
  {
    this.getSites().remove(site);
  } 

@Entity
@Table(name = "site")
public class Site
{
  @ManyToOne
  @JoinColumn(name="server_id")
  protected Server server;
}

Site site1 = siteDAO.find(1);
Site site2 = siteDAO.find(2);

Server server1 = serverDAO.find(1);

server1.addSite(site1);
server1.addSite(site2);

serverDAO.merge(server1);