Hibernate Mapping Strategy :Table per subclass

Hibernate Mapping Strategy :Table per subclass 

As the name suggest, we have table define for each class(abstract, interface or concrete).

Example :

As in this Example, we have one superclass Employee and 2 subclass PermanentEmployee and ContractEmployee. At the database level, we have 3 tables Employee, PermanentEmployee and ContractEmployee. Here Primary key of Employee table acts as both primary key and foreign key for PermanentEmployee and ContractEmployee tables.

Java Class :

Employee.java
package com.demo.example1.pojo;

public class Employee {

private String name;
private String email;
private Long empId;

public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Long getEmpId() {
return empId;
}
public void setEmpId(Long empId) {
this.empId = empId;
}
}

PermanentEmployee.java

package com.demo.example1.pojo;

import java.util.Date;

public class PermanentEmployee extends Employee {

private String designation;

private Date joiningDate;

public String getDesignation() {

return designation;

}

public void setDesignation(String designation) {

this.designation = designation;

}

public Date getJoiningDate() {

return joiningDate;

}

public void setJoiningDate(Date joiningDate) {

this.joiningDate = joiningDate;

}

}

ContractEmployee.java

package com.demo.example1.pojo;

import java.util.Date;

public class ContractEmployee extends Employee {

private Date contractDate;

public Date getContractDate() {

return contractDate;

}

public void setContractDate(Date contractDate) {

this.contractDate = contractDate;

}

}

hbm file : 

<hibernate-mapping package="com.demo.example1.pojo">

<class name="Employee" table="employee">

<id name="empId" column="empId" type="long">

<generator class="identity"/>

</id>

<property name="name" column="name" type="string"></property>

<property name="email" column="email"  type="string" ></property>

<joined-subclass name="PermanentEmployee" table="PermanentEmployee" >

<key column="perm_emp_id"/>

<property name="designation" column="designation" />

<property name="joiningDate" column="joiningDate" type="timestamp" />

</joined-subclass>

<joined-subclass name="ContractEmployee" table="ContractEmployee">

<key column="contract_emp_id"/>

<property name="contractDate" column="contractDate" type="timestamp" />

</joined-subclass>

</class>

</hibernate-mapping>

Main Class :

package com.demo.example1;

import java.util.List;

import org.hibernate.Session;

import org.hibernate.Transaction;

import com.demo.example1.pojo.ContractEmployee;

import com.demo.example1.pojo.Employee;

import com.demo.example1.pojo.PermanentEmployee;

public class Test {

public static void main(String[] args) {

// First unit of work

Session session = HibernateUtil.getSessionFactory().openSession();

Transaction tx = session.beginTransaction();

PermanentEmployee pm = new PermanentEmployee();

pm.setDesignation("perm_desg") ;

pm.setEmail("simon@test.com");

pm.setName("Simon") ;

session.save(pm);

ContractEmployee cm = new ContractEmployee();

cm.setEmail("nathen@test.com");

cm.setName("Nathen") ;

session.save(cm) ;

List<Employee> em = session.createQuery("from Employee").list();

System.out.println(em.size());

tx.commit();

session.close();

HibernateUtil.shutdown();

}

}

Output : 

Hibernate: 

    /* insert com.demo.example1.pojo.PermanentEmployee

        */ insert 

        into

            employee

            (name, email) 

        values

            (?, ?)

Hibernate: 

    /* insert com.demo.example1.pojo.PermanentEmployee

        */ insert 

        into

            PermanentEmployee

            (designation, joiningDate, perm_emp_id) 

        values

            (?, ?, ?)

Hibernate: 

    /* insert com.demo.example1.pojo.ContractEmployee

        */ insert 

        into

            employee

            (name, email) 

        values

            (?, ?)

Hibernate: 

    /* insert com.demo.example1.pojo.ContractEmployee

        */ insert 

        into

            ContractEmployee

            (contractDate, contract_emp_id) 

        values

            (?, ?)

Hibernate: 

    /* 

from

    Employee */ select

        employee0_.empId as empId1_,

        employee0_.name as name1_,

        employee0_.email as email1_,

        employee0_1_.designation as designat2_2_,

        employee0_1_.joiningDate as joiningD3_2_,

        employee0_2_.contractDate as contract2_3_,

        case 

            when employee0_1_.perm_emp_id is not null then 1 

            when employee0_2_.contract_emp_id is not null then 2 

            when employee0_.empId is not null then 0 

        end as clazz_ 

    from

        employee employee0_ 

    left outer join

        PermanentEmployee employee0_1_ 

            on employee0_.empId=employee0_1_.perm_emp_id 

    left outer join

        ContractEmployee employee0_2_ 

            on employee0_.empId=employee0_2_.contract_emp_id

2

Advantage : 

1. The SQL schema is Normalized.
2. A polymorphic association to a particular subclass is represented as a foreign key referencing the table of that subclass 

Disadvantage : 

1. Relies on outer join when querying for Employee class as shown in output .