Hibernate parent/child and foreign key relationships mapping

[Prince Manchanda]

I am trying to understand some concept of O/R mapping using Hibernate by taking some pseudo example.

I have two entities Student and Instructor. Both of these have some common properties like first name, last name, address etc.
Also these can have specific properties like student has roll number while instructor has employee code. To model this I have created following classes:

public abstract class BeanClass {

private Date createdDate;
private Date modifiedDate;
 
}
public abstract class Person extends BeanClass implements Serializable {

private String personId;
private String firstName;
private String lastName;
private Set<Address> address;
}
    

Person class is abstract since I want to use it's specialised children.

public class Student extends Person {
 
private String rollNumber;
private CourseClass courseClass; 
}
 
public class Instructor extends Person {
 
private String employeeNumber;
 
private Person person;
}
    

I have ommitted the getter/setter and other methods.
I have following mapping files:
Instructor.hbm.xml

 
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 

<hibernate-mapping>
<class name="com.prince.school.db.dto.Teacher" table="teacher">
<id name="employeeNumber" column="employee_id">
<generator class="uuid"/>
</id>

<property name="createdDate" column="created_date"/>
<property name="modifiedDate" column="modified_date"/>


<many-to-one name="person" column="person_id" not-null="true"/>
<!--<one-to-one name="person" class="com.prince.school.db.dto.Person" lazy="false"/>-->
 
</class>
</hibernate-mapping>
   

Person.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 

<hibernate-mapping>
<class name="com.prince.school.db.dto.Person" table="person">
<id name="personId" column="person_id">
<generator class="uuid"/>
</id>
<property name="firstName" column="first_name"/>
<property name="lastName" column="last_name"/>
<property name="createdDate" column="created_date"/>
<property name="modifiedDate" column="modified_date"/>


<set name="address" table="address" lazy="false" cascade="save-update" inverse="true">
<key column="person_id" />
<one-to-many class="com.prince.school.db.dto.Address"/>
</set>


<set name="teacher" inverse="true" cascade="all-delete-orphan" lazy="false">
    <key column="person_id"/>
    <one-to-many class="com.prince.school.db.dto.Teacher"/>
</set>
 

 
</class>
</hibernate-mapping>
   

And similarly Student.hbm.xml and Address.hbm.xml.

My database schema is

ij> describe person;
COLUMN_NAME         |TYPE_NAME|DEC&|NUM&|COLUM&|COLUMN_DEF|CHAR_OCTE&|IS_NULL&
------------------------------------------------------------------------------
PERSON_ID           |VARCHAR  |NULL|NULL|10    |NULL      |20        |NO
FIRST_NAME          |VARCHAR  |NULL|NULL|30    |NULL      |60        |NO
LAST_NAME           |VARCHAR  |NULL|NULL|30    |NULL      |60        |YES
CREATED_DATE        |DATE     |0   |10  |10    |NULL      |NULL      |NO
MODIFIED_DATE       |DATE     |0   |10  |10    |NULL      |NULL      |YES
 
ij> describe instructor;
COLUMN_NAME         |TYPE_NAME|DEC&|NUM&|COLUM&|COLUMN_DEF|CHAR_OCTE&|IS_NULL&
------------------------------------------------------------------------------
EMPLOYEE_ID         |VARCHAR  |NULL|NULL|10    |NULL      |20        |NO
PERSON_ID           |VARCHAR  |NULL|NULL|10    |NULL      |20        |NO
COURSE_CLASS_ID     |VARCHAR  |NULL|NULL|10    |NULL      |20        |NO
CREATED_DATE        |DATE     |0   |10  |10    |NULL      |NULL      |NO
MODIFIED_DATE       |DATE     |0   |10  |10    |NULL      |NULL      |YES
   

Field person_id in teacher is having foreign key relationship with table person.

Now my problem is that if i try to load data for Instructor, I am not able to get the records from the Person table. I have tried HibernateTemplate's loadAll(Instructor) method and find(query) method

Am I doing something wrong in modelling the entities either in Object model or data model side. Or is there anything wrong in my hibernate mappings.

Also I am using derby as database and Spring's DAO framework.

[Mark Spritzler]

"P Manchanda",
Welcome to the JavaRanch.

We're a friendly group, but we do require members to have valid display names.

Display names must be two words: your first name, a space, then your last name. Fictitious names are not allowed.

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[Mark Spritzler]

Well, I am actually seeing two different concepts in your "mapping" I see a one-to-many in the database between two tables, and in Java I see an inheritance. Both require mapping and bother require different mapping for each part.

I highly recommend, while learning that you split those two totally different concepts with two totally different examples.

I also highly recommend for examples and learning to use the Caveat Emptor sample Hibernate app. Which can be downloaded from www.hibernate.org

Good Luck

Mark

[Nakata kokuyo]

Mark,
do you mind to share with us how would you handle the case of P Manchanda

beside, Manchanda, can you specifically point out the relation among object ?

for instance : is student have m:n relation to instructor ?

thanks for guidance

[ April 11, 2008: Message edited by: Nakata kokuyo ]
[ April 11, 2008: Message edited by: Nakata kokuyo ]

[Prince Manchanda]

Thanks for the response guys.
Mark can you elaborate bit more on
Both require mapping and bother require different mapping for each part.


It would be great if you can find some time to post a sample mapping file or do you feel there is a better way to create an object model and data model for this scenerio.

Nakata kokuyo
There can be many to many relationship between student and instructor.

[Mark Spritzler]

Here is the Hibernate docs on Inheritence mapping

http://www.hibernate.org/hib_docs/v3/reference/en/html/inheritance.html

Here is the Hibernate docs to Association mapping

http://www.hibernate.org/hib_docs/v3/reference/en/html/associations.html

They are two different chapters.

So in the example of Student/Instructor the parent to child relationship would be mapped via an Association, and the fk id, when related to shared data between the abstract and subclasses would be mapped using the Inheritence mapping techiques.

Mark