I'm a newbie to JPA and Hibernate. I was able to set up some small basic examples as expected. Now I'm trying to use it for a first real world project. Studying the Hibernate best practices I found that you should avoid many-to-many relationships. I found relating questions here and I do understand the concept why not to use it but I'm not understanding how it should be implemented.
So when I have the often used example of a user that can the part of many groups and a group that does have many users, how to implement that. So this is a many-to-many relationship. But I should not use many-to-many, as far as I understood because it is very likely that I will need other information in the future, like a specific role a user has in a group or a date when he joined. So I use a two one-to-many relationships and a joining table which doesnt only contains the ids but also will contain additional information like role or date. Is that right? And then neither the class group has a property users nor the class users has a property groups both have a property joinTableEntries?
Did I get the concept right so far?
@Entity
@Table(name="user")
public class User {
private int userId;
private String username;
private Set<JTUserGroup> jtUserGroupSet=new HashSet<JTUserGroup>(0);
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "userid")
public int getUserId()
{
return this.userId;
}
public void setUserId(int userId) {
this.userId = userId;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
@OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
public Set<JTUserGroup> getJtUserGroupSet() {
return jtUserGroupSet;
}
public void setJtUserGroupSet(Set<JTUserGroup> jtUserGroupSet) {
this.jtUserGroupSet = jtUserGroupSet;
}
}
@Entity
@Table(name = "forumgroup")
public class Group {
private int groupId;
private String groupname;
private Set<JTUserGroup> jtUserGroupSet=new HashSet<JTUserGroup>(0);
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "groupid")
public int getGroupId() {
return groupId;
}
public void setGroupId(int groupId) {
this.groupId = groupId;
}
public String getGroupname() {
return groupname;
}
public void setGroupname(String groupname) {
this.groupname = groupname;
}
@OneToMany(fetch = FetchType.LAZY, mappedBy = "group")
public Set<JTUserGroup> getJtUserGroupSet() {
return jtUserGroupSet;
}
public void setJtUserGroupSet(Set<JTUserGroup> jtUserGroupSet) {
this.jtUserGroupSet = jtUserGroupSet;
}
}
@Entity
@Table(name = "jtusergroup")
public class JTUserGroup {
private int joinId;
private User user;
private Group group;`enter code here`
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getJoinId() {
return joinId;
}
public void setJoinId(int joinId) {
this.joinId = joinId;
}
@ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JoinColumn(name = "userid", nullable = false)
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
@ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JoinColumn(name = "groupid", nullable = false)
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}
}
and then to use these I add a new user to a group like this:
User user1=new User();
User user2=new User();
User user3=new User();
Group group1=new Group();
Group group2=new Group();
user1.setUsername("Mark");
user2.setUsername("Anton");
user3.setUsername("Maggy");
group1.setGroupname("Mark_Maggy");
group2.setGroupname("Maggy_Anton");
JTUserGroup jt1=new JTUserGroup();
jt1.setGroup(group1);
jt1.setUser(user1);
JTUserGroup jt2=new JTUserGroup();
jt2.setGroup(group1);
jt2.setUser(user3);
JTUserGroup jt3=new JTUserGroup();
jt3.setGroup(group2);
jt3.setUser(user3);
JTUserGroup jt4=new JTUserGroup();
jt4.setGroup(group2);
jt4.setUser(user2);
GenericDAO<JTUserGroup> jtDao=new GenericDAO<JTUserGroup>();
jtDao.beginTransaction();
jtDao.insert(jt1);
jtDao.insert(jt2);
jtDao.insert(jt3);
jtDao.insert(jt4);
jtDao.commit();
