Hibernate association mapping

[Girish Kumar Prabhakar]

Hi All,

I have 2 tables : Employee and EmployeeRole.
Both tables have their own Primary Keys. Employee has EmpId and EmployeeRole has EmpRoleId.

How can i make one to one mapping between these 2 tables in Hibernate.

Please guide me with both the hbm files.

Thanks in advance.

[ Edited to have a more meaningful subject line - Paul Sturrock ]
[ December 09, 2008: Message edited by: Paul Sturrock ]

[Jaikiran Pai]

Girish,

Welcome to JavaRanch

You will find a lot of information in the Hibernate reference guide. It also has a chapter for one-to-one mappings.

[Girish Kumar Prabhakar]

Hi JaiKiran, Thanks for your reply, but i have already read that. I think you have not seen my problem carefully. I have 2 primary Keys here. Not one PK as FK in other table. So how do i map these. My tables are : Employee : EmpId (PK) EmpName emailId EmployeeRole : EmpRoleId(PK) EmpId ( FK from Employee table) RoleId . Now can you help me.

[Paul Sturrock]

I have 2 primary Keys here.

Not sure why this is relevant? They are primary keys for different entities. (after all, you can't have two primary keys for one entity).

Without knowing the cardinality of the relationship I can't give you an example. But the section on associations covers all the common ones (with examples).

[Girish Kumar Prabhakar]

Hi Paul, I dont have 2 Primary Keys for one entity. I have 2 entites : Employee and Role. And both have there own Primary Keys as EmpId and EmpRoleId respectively. Now can you tell me how to create one to one mapping in this scenario. Thanks

[Paul Sturrock]

Originally posted by Girish Kumar Prabhakar:
Hi Paul, I dont have 2 Primary Keys for one entity. I have 2 entites : Employee and Role. And both have there own Primary Keys as EmpId and EmpRoleId respectively. Now can you tell me how to create one to one mapping in this scenario. Thanks

Yes I can. This document contains examples of all common association mappings, including an example of one-to-one.

[Chandra Bhatt]

Hi Girish,

To map between two entities you need to find out the cardinality. What is
relationship between Employee and EmployeeRole entities.

I may suppose following associations:
- One Employee can have multiple roles.
- One Employee can have exactly one role.

If two entities share the primary key you can have one-to-one relationship
in either unidirectional or bidirectional.

Unidirectional: If you are given Employee object, you can tell what role
is assigned to him. But EmployeeRole doesn't tell you which Employee does
it belong to.

BiDirectional: In both the way you can find out each other entity, as both
will have reference to each other.

As you said both the tables have primary keys EmployeeId and EmployeeRoleId.
I do not find any association building clue using both, as individual
represent their own entities.

[Girish Kumar Prabhakar]

Hi Chandra,

Thanks for your reply.

Here One Employee can have exactly one Role.
I am attaching the hbm files of both.
Please have a look and guide me.

Employee.hbm.xml

 
<hibernate-mapping>
 
<class name="com.Employee" table="EMP_MST">
<id name="empId" column="empId">
<generator class="increment"/>
</id>

<property name="empName" column="empName"/>
<property name="mgrId" column="mgrId"/>
<property name="emailId" column="emailId"/>
<property name="shortId" column="shortId"/>
<property name="deptId" column="deptId"/>
<one-to-one name="role" ></one-to-one>
</class>
 
</hibernate-mapping>

EmployeeRole.hbm.xml

 
<hibernate-mapping>
 
<class name="com.EmpRole" table="EMP_ROLE">
<id name="empId" column="empId">
<generator class="foreign">
<param name="property">employee</param>
</generator>
</id>

<property name="roleId" column="roleId"/>
<one-to-one name="employee" constrained="true"/>

</class>
 
</hibernate-mapping>    

and i am trying to do :

Session session = HibernateConnection.currentSession();
session.beginTransaction();

Employee emp = new Employee();
emp.setEmpId(1000);

EmpRole role = new EmpRole();
Employee emp1 = new Employee();
emp1.setEmpId(1000);
role.setEmployee(emp1);
role.setRoleId(830);
//role.setEmpId(100);
emp.setRole(role);

session.saveOrUpdate(emp);

session.getTransaction().commit();

HibernateConnection.closeSession();

}

I only want to update in EMployeeRole and not in Employee.

Can you suggest any thing.

Thanks

[ Edited to use code tags - Paul Sturrock ]
[ December 10, 2008: Message edited by: Paul Sturrock ]

[Chandra Bhatt]

Hi Girish,

/*
 * Employee.java
 *
 * Created on December 11, 2008, 12:47 AM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package com.javaranch.beans;
import javax.persistence.*;
 
/**
 *
 * @author Chandra
 */
@Entity
@Table(name="EMPLOYEE_1")
public class Employee {
    
    /** Creates a new instance of Employee */
    public Employee() {
    }
    
    private Integer empId;
    private String empName;
    private String emailId;
    private Integer mgrId;
    private EmployeeRole role;
    
    public String getEmpName() {
        return empName;
    }
 
    public void setEmpName(String empName) {
        this.empName = empName;
    }
 
    public String getEmailId() {
        return emailId;
    }
 
    public void setEmailId(String emailId) {
        this.emailId = emailId;
    }
 
    public Integer getMgrId() {
        return mgrId;
    }
 
    public void setMgrId(Integer mgrId) {
        this.mgrId = mgrId;
    }
    @OneToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="roleId")
    public EmployeeRole getEmployeeRole() {
        return role;
    }
 
    public void setEmployeeRole(EmployeeRole role) {
        this.role = role;
    }
    @Id @GeneratedValue
    public Integer getEmpId() {
        return empId;
    }
 
    public void setEmpId(Integer empId) {
        this.empId = empId;
    }
}

/*
 * EmployeeRole.java
 *
 * Created on December 11, 2008, 12:47 AM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package com.javaranch.beans;
 
import javax.persistence.*;
 
@Entity
@Table(name="EMPLOYEE_ROLE")
public class EmployeeRole {
    
    /**
     * Creates a new instance of EmployeeRole
     */
    public EmployeeRole() {
    }
    
    
    private Integer roleId;
    private Employee emp;
 
    @Id 
    public Integer getRoleId() {
        return roleId;
    }
 
    public void setRoleId(Integer roleId) {
        this.roleId = roleId;
    }
    @OneToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="empId")
    public Employee getEmployee() {
        return emp;
    }
 
    public void setEmployee(Employee emp) {
        this.emp = emp;
    }
    
}

alter table EMPLOYEE_1 drop foreign key FK26EDEEE0C5E01499
alter table EMPLOYEE_ROLE drop foreign key FK429957676E2224D5
drop table if exists EMPLOYEE_1
drop table if exists EMPLOYEE_ROLE
create table EMPLOYEE_1 (empId integer not null auto_increment, emailId varchar(255), empName varchar(255), mgrId integer, roleId integer, primary key (empId))
create table EMPLOYEE_ROLE (roleId integer not null, empId integer, primary key (roleId))
alter table EMPLOYEE_1 add index FK26EDEEE0C5E01499 (roleId), add constraint FK26EDEEE0C5E01499 foreign key (roleId) references EMPLOYEE_ROLE (roleId)
alter table EMPLOYEE_ROLE add index FK429957676E2224D5 (empId), add constraint FK429957676E2224D5 foreign key (empId) references EMPLOYEE_1 (empId)

com.javaranch.beans.Employee emp = new com.javaranch.beans.Employee();
        emp.setEmailId("[e-mail]");
        emp.setEmpName("Davidson");
        emp.setMgrId(1201);        
        EmployeeRole role = new EmployeeRole();
        role.setRoleId(56);
        role.setEmployee(emp);
        emp.setEmployeeRole(role);
        Session session = factory.openSession();
        Transaction t = session.beginTransaction();
        session.save(role);
        t.commit();

Does it adhere what your requirement says?

[Girish Kumar Prabhakar]

Hi Chandra,

Thanks for your reply, but i have different requirement, hope you can guide me through this.

I already have a row in Employee table with empId as 1000 empName as Girish and so on. Similarly i have a row corresponding to this in EmployeeRole table with empId as 1000 and roleId as 100.

I only want to update the EmployeeRole table and set roleId as 200 for empId as 1000. There will be no update or insert in Employee table.

Now can you guide on the same.

Thanks in advance.

[Chandra Bhatt]

I only want to update the EmployeeRole table and set roleId as 200 for empId as 1000. There will be no update or insert in Employee table.

For this you want to set property on the Employee object as

emp.setEmpId(1000);
emp.setRole(200);
session.saveOrUpdate(emp);

Is this a way what you want?

[Chandra Bhatt]

    
/*
 * EMP.java
 *
 * Created on December 11, 2008, 8:30 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package com.javaranch.beans;
 
import javax.persistence.*;
 
/**
 *
 * @author Chandra
 */
@Entity
public class EMP {
    
    /** Creates a new instance of EMP */
    public EMP() {
    }
    
    private Integer empId;
    private String name;
    
    @Id
    public Integer getEmpId() {
        return empId;
    }
 
    public void setEmpId(Integer empId) {
        this.empId = empId;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }    
}

    
/*
 * ROLE.java
 *
 * Created on December 11, 2008, 8:30 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
package com.javaranch.beans;
 
import javax.persistence.*;
 
/**
 *
 * @author Chandra
 */
@Entity
@Table(
name="ROLE", 
uniqueConstraints=
@UniqueConstraint(columnNames={"empId"}))
public class ROLE {
    
    /** Creates a new instance of ROLE */
    public ROLE() {
    }
    
    private Integer id;
    private Integer roleId;
    private EMP emp;
 
    public Integer getRoleId() {
        return roleId;
    }
 
    public void setRoleId(Integer roleId) {
        this.roleId = roleId;
    }
    @OneToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="empId")
    public EMP getEmp() {
        return emp;
    }
    
    public void setEmp(EMP emp) {
        this.emp = emp;
    }
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
 
    public void setId(Integer id) {
        this.id = id;
    }
}
 

Below is auto-generated table schema:

alter table ROLE drop foreign key FK2678764DC4F3E7
drop table if exists EMP
drop table if exists ROLE
create table EMP (empId integer not null, name varchar(255), primary key (empId))
create table ROLE (id integer not null auto_increment, roleId integer, empId integer, primary key (id), unique (empId))
alter table ROLE add index FK2678764DC4F3E7 (empId), add constraint FK2678764DC4F3E7 foreign key (empId) references EMP (empId)

    
com.javaranch.beans.EMP emp1 = new com.javaranch.beans.EMP();
        com.javaranch.beans.EMP emp2 = new com.javaranch.beans.EMP();
        
        emp1.setEmpId(2500);
        emp1.setName("Davidson");
        
        emp2.setEmpId(4000);
        emp2.setName("Chandra");
       
        
        Session session = factory.openSession();
        Transaction t = session.beginTransaction();
        session.saveOrUpdate(emp1);
        session.saveOrUpdate(emp2);             
        
        com.javaranch.beans.ROLE role = new com.javaranch.beans.ROLE();
        role.setId(2); // where empId=4000
        role.setEmp(emp2);
        role.setRoleId(133); //change the role of employeeId=4000
 
        session.saveOrUpdate(role);
        t.commit();        

[Chandra Bhatt]

Girish,

I forgot to put details.

I have put unique constraint on empId of the ROLE table as said there can be
only one role assigned to an Employee.

Id field of ROLE schema is primary key.
We can't take roleId to be primary key because same role can be assigned
to more than one Employee, so it is a separate column. (values to this
column will be redundant (not unique) as per your requirement)

[Girish Kumar Prabhakar]

Hi Chandra,

Thanks again for your reply. The code which you gave on the basis of that i have written mine, here it is:


Session session = HibernateConnection.currentSession();
session.beginTransaction();

Employee emp = new Employee();
emp.setEmpId(1000);
emp.setEmpName("Girish");

session.saveOrUpdate(emp);
EmpRole role = new EmpRole();
role.setEmpRoleId(1);
role.setRoleId(147);
role.setEmployee(emp);
session.saveOrUpdate(role);

session.getTransaction().commit();

HibernateConnection.closeSession();


Now this code updates the EmployeeRole table and sets the new role as required.

BUT the problem which i face now in this code is that empId for this newrole in EmployeeRole table is becoming null.

What to do now???

Also i have not set empId in EmployeeRole table as Unique constraint.
will that affect this.

Waiting for your reply.

Thanks a tonn.

[Chandra Bhatt]

Girish,

BUT the problem which i face now in this code is that empId for this newrole in EmployeeRole table is becoming null.

It should not happen because there is a mapping:

 
...   
@OneToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="empId")
    public EMP getEmp() {
        return emp;
    }
...

It works for me.
role.setEmp(emp); and saving or updating this role, saves the empId in
the employee role table.

Also i have not set empId in EmployeeRole table as Unique constraint.
will that affect this.

No! I just made that UniqueConstraint to suffice the need that one employee
can be assigned at most one role as your requirement says.

Hope this helps you.

[Chandra Bhatt]

BUT the problem which i face now in this code is that empId for this newrole in EmployeeRole table is becoming null.

Are you following the schema that I say:
id, roleId, empId, ... for EmployeeRole table?


It takes null as default value for empId in EmployeeRole table.
empId is foreign key that points to PK of Employee table of course.
It should work in this regard.