Hibernate - How to do associations with join tables?

[Marcin Mogiela]

Hi,

There is a "student" table and a "course" table (many to many relationship)
There is also a "grade" that I wanted to join with the "student_course" join table. (many one)

How can I do it in Hibernate? I am confused because "student_course" join table will be generated by Hibernate through @ManyToMany annotation


Thanks,
Marcin

[Cameron Wallace McKenzie]

This thread discussed a very similar issue:

JavaRanch Thread on Adding Properties to Joined Many to Many Table Example

Not to ruin your enjoyment of reading through that long thread, I think an example of a program that adds a property to a class representing the join table of a many-to-many relationship in Hibernate3 can be found here:

hibernate annotations composite mapping

Post back, and let us know if this puts you in the right direction.

-Cameron McKenzie

[Marcin Mogiela]

Hi Cameron, it's a pleasure talking to you

The example from http://java-aap.blogspot.com/2006/04/hibernate-annotations-composite.html is a little too advanced for me.
I have only two days experience with hibernate and have read half and a little tutorial from your site http://www.hiberbook.com/HiberBookWeb/learn.jsp

All I understood from the example was that if a join table has its own properties a new POJO should be created

Now for the magic:
I've added 2 extra getters and setters for Product and Item to make it easier to get or set a product or item on your primary key. I added to extra columns to make hibernate think there is a Product mapped to this table (and an Item) but hibernate never updates or inserts this property (becuase it doensn't exists in our db.) But when hibernate does the getProduct() method it will retrieve the Product from our composite foreign key and all will work fine!

I don't understand this magic. Right now it seems to me like a hack

Thanks
[ May 19, 2008: Message edited by: Marcin Mogiela ]

[Pradeepta Bhattacharya]

Did anyone found out any solution for this situation?
[ June 22, 2008: Message edited by: Pradeepta Bhattacharya ]

[Marcin Mogiela]

Hi Pradeepta,

I did something like below, though it may not be the optimal solution.

If anybody had better solution for this problem or found any flaw in the problem itself I would highly appreciate any suggestions.

First of all I introduced the join class Enrollment.

public class Enrollment {
private Long id;
 
private Student student;
 
private Course course;
 
private Set<Grade> grades = new HashSet<Grade>();
 
        // getters and setters

From Enrollment.hbm.xml

<hibernate-mapping>
<class name="truant.model.Enrollment" table="ENROLLMENT">
<id name="id" column="ENROLLMENT_ID">
<generator class="increment" />
</id>
 
<many-to-one name="student" class="truant.model.Student"
column="STUDENT_ID" not-null="true" cascade="save-update" />
 
<many-to-one name="course" class="truant.model.Course"
column="COURSE_ID" not-null="true" cascade="save-update" />
 
<set name="grades" inverse="true" cascade="save-update">
<key column="GRADE_ID" />
<one-to-many class="truant.model.Grade" />
</set>
</class>
</hibernate-mapping>

From Course.hbm.xml

<set name="enrollments" inverse="true" cascade="save-update">
<key column="COURSE_ID" />
<one-to-many class="truant.model.Enrollment" />
</set>

From Student.hbm.xml

<set name="enrollments" inverse="true" cascade="save-update">
<key column="STUDENT_ID" />
<one-to-many class="truant.model.Enrollment" />
</set>

From Grade.hbm.xml

<many-to-one name="enrollment" class="truant.model.Enrollment"
column="STUDENT_COURSE_ID" />

[Jignesh Patel]

Originally posted by Marcin Mogiela:


All I understood from the example was that if a join table has its own properties a new POJO should be created

Cameron,
what if we don't have extra properties in join table. Do we still need extra POJO for join table? I have a working example without have new POJO for join table though I have not created compound primary key for join table.

So I have threet ables Group, Receiver and receiver_group.
While I have two classes Group and Reciver but when I insert the data the join table also gets updated.

[Bauyrzhan Matyganov]

*********************************************************
how to write a setter and getter methods for Student and Course fields in Enrolment class?

Marcin Mogiela wrote:Hi Pradeepta,

I did something like below, though it may not be the optimal solution.

If anybody had better solution for this problem or found any flaw in the problem itself I would highly appreciate any suggestions.

First of all I introduced the join class Enrollment.

public class Enrollment {
private Long id;
 
private Student student;
 
private Course course;
 
private Set<Grade> grades = new HashSet<Grade>();
 
        // getters and setters

From Enrollment.hbm.xml

<hibernate-mapping>
<class name="truant.model.Enrollment" table="ENROLLMENT">
<id name="id" column="ENROLLMENT_ID">
<generator class="increment" />
</id>
 
<many-to-one name="student" class="truant.model.Student"
column="STUDENT_ID" not-null="true" cascade="save-update" />
 
<many-to-one name="course" class="truant.model.Course"
column="COURSE_ID" not-null="true" cascade="save-update" />
 
<set name="grades" inverse="true" cascade="save-update">
<key column="GRADE_ID" />
<one-to-many class="truant.model.Grade" />
</set>
</class>
</hibernate-mapping>

From Course.hbm.xml

<set name="enrollments" inverse="true" cascade="save-update">
<key column="COURSE_ID" />
<one-to-many class="truant.model.Enrollment" />
</set>

From Student.hbm.xml

<set name="enrollments" inverse="true" cascade="save-update">
<key column="STUDENT_ID" />
<one-to-many class="truant.model.Enrollment" />
</set>

From Grade.hbm.xml

<many-to-one name="enrollment" class="truant.model.Enrollment"
column="STUDENT_COURSE_ID" />