Error in Many to one relation - google-app-engine

Question

My Organization entity
#Entity
public class Organization implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Key key;
private String name;
private String type;
private byte image;
#OneToMany(cascade=CascadeType.MERGE)
#JoinColumn(name="ORGANIZATION_ID")
private List<User> admin;
My User entity
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Key key;
private String score;
private boolean online;
private String resume;
public Status status;
public enum Status {
ACTIVE, INACTIVE, VERIFIED, NOT_VERIFIED , BANNED
};
#ManyToOne(cascade=CascadeType.MERGE)
#JoinColumn(name="ORGANIZATION_ID")
private Organization organization;
#Persistent
private User_personal user_p;
public User_personal getUser_personal(){
return user_p;
}
public void setUser_personal(User_personal user_p) {
this.user_p = user_p;
}
#OneToMany(mappedBy = "user", cascade=CascadeType.MERGE)
private List<Project> projects;
I got the one-Many relation between User and Projects correctly but not working for User and Organization(many-one).I am getting error like this
WARNING: /OrganizationServlet
javax.persistence.PersistenceException: Detected attempt to establish
Organization(no-id-yet) as the parent of User(4793870697103360) but the
entity identified by User(4793870697103360) has already been persisted
without a parent. A parent cannot be established or changed once an object
has been persisted.at...
showing error at em.getTransaction().commit();.
My servlet is
protected void doPost(HttpServletRequest request, HttpServletResponse
response) throws ServletException, IOException {
// TODO Auto-generated method stub
HashMap<String,String> map = Request_to_map.getBody(request);
boolean validToken = JWT.parseJWT(request.getHeader("Access-token")
,map.get("email"));
JsonObject output = new JsonObject();
List<User> organization_admin = new ArrayList<User>();
if(validToken == true){
EntityManager em;
em = EMF.get().createEntityManager();
String organizationName = map.get("name");
String type = map.get("type");
byte image = 0;
if(map.get("image")!=null)
{
image = Byte.valueOf(map.get("image"));
}
String email = map.get("email");
if(organizationName==null||type==null||email==null||map.get("image")==null)
{
throw new IllegalArgumentException("please fill required
details");
}
try{
em.getTransaction().begin();
User user = User.find(email, em);
if(user!=null)
{
Organization.org_status status= org_status.ACTIVE;
Organization organization = new
Organization(organizationName, type,image,status);
user.setOrganization(organization);
organization_admin = organization.getAdmin();
if(organization_admin == null)
{
organization_admin = new ArrayList<User>();
}
organization_admin .add(user);
organization.setAdmin(organization_admin);
em.persist(organization);
em.persist(user);
output.addProperty("message", "done");
em.getTransaction().commit();
}
else
output.addProperty("message","No such User found.Please
check details provided");
}
finally{
if(em.getTransaction().isActive())
em.getTransaction().rollback();
// em.close();
}
}
else
output.addProperty(Constants.MESSAGE,
Constants.TokenNotAuthenticated);
response.setContentType("application/Json");
response.getWriter().println(output);
}
Can anyone help me in getting this? When user is created I am getting ORGANIZATION_ID as a column but cant create entity of organization.I dont think joins are to be used as GAE doesn't allow it.