Was wondering what the folks think about the best practice.
Maybe this is a more object model question than an ORM type of question.

Lets say there is a rule that says 1 Student has Many Cars.
But 1 Car can belong only to 1 Student.

so..

Student has a one-to-many relationship to Car

What are the merrits for this to be a bi-directional relationship?

ie..
if the Student.java

//
String name;
Set <Car> cars = new HashSet<Car>();
//getters and setters


Car.java
//
String brand;
Student student;
// getters and setters.

By doing this it is possible to declare the mappings and have in our Studen.hbm.xml

<set name="cars">
<key column="student_id"/>
<one-to-many class="Car"/>
</set>

In Car.hbm.xml

<many-to-one name="student" column="student_id"/>

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If the use case says we need to display all the cars associated for a particular student.. if we keep the bi-directionality (ie Student knows about its cars and a Car knows about its Student)
we could simply say.. studentX.getCars();
...
However if we keep a uni-directional relationship.. that is dropping the
Set <Car> cars = new HashSet<Car>(); from Student.java
and dropping this mapping from Student.hbm.xml
And if we had a DAO ran a method that would have some HQL like
("from Car c where c.student = student) and this returned a Collection of all the cars that belong to a particular student.
Thus from the 2 above strategies we are achieving the best result..
However...what is good practice?? to maintain bi-directionality when a domain object needs to know about its relationship to its children?
or to use uni-directionality and write hql to fetch collections of its parent children??

When do we need bidirectionality??? and when is not necessary??

Thanks..

Have a look at the ref docs, chapter 24, "Best Practices". In particular, the last item, "prefer bidirectional associations", may have some words of advice for you.

It's quite easy to find: go to the end of the ref docs. Voila.

Thanks..tenwit..
however the reference..though gives me an insight..it sounds like bidirectional is the better approach...
Could any one give me a reason why this is a best practice as oppose to using unidirectional only??
10nks

They're equally good, except that it's more awkward to search using unidrection associations. If you're prepared to deal with the uglier query code, then it doesn't matter. That's why the ref docs just say to prefer bidirectional, rather than "don't use unidirectional".

In over 90% of cases, a lazily-loaded bidirectional association is just as efficient as the unidirectional version, and it's always more convenient.