A design question related to generic dao pattern

Question

I follow this link https://www.hibernate.org/328.html to design my DAO layer using jpa/ hibernate. And basically it works ok. However, I encounter a dilemma regarding to persistence relation between domain objects.
My classes include
User <--(many to many)--> Group
Tables are USERS, USERS_GROUPS, GROUPS
In User class, I have properties
     @JoinTable(name="USERS_GROUPS",
          joinColumns=
          @JoinColumn(name="USER_ID"),
          inverseJoinColumns=
          @JoinColumn(name="GROUP_ID")
     private List<Group> groups = new ArrayList<Group>();
     public User(String id, String account, String name, String password){
          this.id = id;
          this.account = account;
          this.name = name;
          this.password = password;
     public List<Group> getGroups(){
          return this.groups;
     public void setGroups(List<Group> groups){
          this.groups = groups;
     }In Group class,
     @ManyToMany(mappedBy="groups",fetch=FetchType.EAGER)
     private List<User> users;  
     public Group(String id, GroupName name){
          this.id = id;
          this.name = name;
     public List<User> getUsers(){
          return this.users;
     public void setUsers(List<User> users){
          this.users = users;
...Now in the database (mysql), I have already had two groups existing. They are
mysql> select * from GROUPS;
+----+------+
| ID | NAME |
+----+------+
| 1  | Root |
| 2  | User |
+----+------+and in the table USERS_GROUPS
mysql> select * from USERS_GROUPS;
+--------------------------------------+----------------+
| USER_ID                               | GROUP_ID |
+--------------------------------------+----------------+
| 1                                               | 1                 |
| 1                                               | 2                 |
+--------------------------------------+----------------+When I want to create a new user object and assig user object with an existing GROUP (e.g. GroupName.User)
At the moment the way how I implement it is done in the DAO layer (e.g. UserDaoImpl). (But what I want is to get this procedure done in the domain object layer i.e. User class, not DAO layer. )
steps:
1.) search the GROUP table (entityManager.cerateQuery(" from Group").getReulstList())
So it will return a list of all groups in GROUPS table.
2.) then check if the group (e.g GroupName.User) I want to assign to the User object existing in the GROUP table or not.
3.) if not existing, create a new Group and add it to a list; then User.setGroups(newList);
or
if the group (to be assigned to User class) already exists, use the group retrieved from the GROUP table and assign this group to the User class (e.g. groupList.add(groupFoundInGroupTable) User.setGroups(groupList))
However, what I want to do is to encapsulate this procedure into User calss. So that in DAO class whilst performing function createUser(), it would only need to do
User user = new User(....);
user.addGroup(GroupName.User); // GroupName is an enum in which it contains two values - Root, User
entityManager.merge(user);Unfortunately, because I need to obtaining Group collections from GROUP table first by using entityManager, which does not exist in the domain object (e.g. User class). So this road looks like blocked.
I am thinking about this issue, but at the moment there is no better solution popped upon my head.
Is there any chance that this procedure can be done more elegantly?
I appreciate any suggestion.
Edited by: shogun1234 on Oct 10, 2009 7:25 PM

Answer 1

You should be able to link your objects using methods in your entities to create an object graph.
When you save your user the associated objects will also be persisted based on the cascade types that you have specified on the relationships between the entities.