i want to save group_code into user table, but why idGroup is saved ?

I have design class group user like this,

how to i can save group_code into table user ?

Code:
@Entity
@Table(name="t_groupuser",
uniqueConstraints = {@UniqueConstraint(
columnNames={"GROUP_CODE",
"PROG_NAME"
})}
)
public class GroupUser implements java.io.Serializable{

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="ID_GROUP")
private int idGroup;

@Column(name="GROUP_CODE", nullable=true,unique=false,length=20)
private String groupCode;

@Column(name="DESCRIPTION", nullable=true,unique=false,length=35)
private String description;

@Column(name="PROG_NAME", nullable=true,unique=false,length=20)
private String groupCode;


Code:
@Entity
@Table(name="t_user",
uniqueConstraints = {@UniqueConstraint(
columnNames={"USER_CODE",
"PROG_NAME"
})}
)
public class User implements java.io.Serializable {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="ID_USER")
private int idUser;

@Column(name="USER_CODE",nullable=false,unique=false, length=15)
private String userCode;

@Column(name="USER_NAME",nullable=false,unique=false, length=25)
private String userName;

@ManyToOne()
@JoinColumn(name="GROUP_CODE")
private GroupUser groupUser;


//get GroupCode
Code:
protected GroupUser getGroupCode(String description){
startOperation();
GroupUser groupUser=null;
try {
groupUser= (GroupUser) con.session.createQuery("from GroupUser where description = :description ")
.setParameter("description",description)
.uniqueResult();
con.session.flush();
}catch(Exception e){e.printStackTrace();
}
con.closeSession();
return groupUser;
}


//..
..
..
show description of group user in combobox
//


//get group code
groupUser = new GroupUser();
groupUserDao=new GroupUserDao();
groupUser=groupUserDao.getGroupCode(cmbGroupUser.getSelectedItem().toString())

Code:
//save data
user = new User();
userDao = new UserDao();
user.setUserCode(txtUserCode.getText().trim());
user.setUserName(txtName.getText().trim());
user.setGroupUser(groupUser);
session.save(user);

Thanks for your help

So now whats your problem? Is it that when you save User object, GroupUser object is also getting updated?