Hi,

I am a newbie to hibernate and am struck with the on-to-many usage. Consider this scenario: I have two tables department and employee and the relation between them is one-to-many.

Department table:
department_id int primary key
department_name not null varchar(100)

Employee table:
employee_id int primary key
employee_name not nul varchar(100)
department_id int foreign key references department(department_id)

The data in the Department table is pre-defined as there will be fixed number of departments in any organization.
Now, I need to create new employee and assign them to a department. For this purpose, I created a jsp page with a text box for name and a drop down list pre-populated with departments. To insert the record, it is pretty straightforward. Using request.getParameter(), I retrieved the values of name and department and assigned them to the Employee object. Using the session.save(), I am able to save the object without using the one-to-many relationship and I suppose it has to be done that way only.

From the examples I found on the web for one-to-many relationship, the parent object is saved first and then the child object.

Now my question is : Should we use one-to-many relationship only when we need to store objects in both the tables?

Regards,
Viswanath

Why don't you do it kind of like this way?:

Your select list of departments is a list of ID's and Labels / Value pairs, like this in HTML


In your code you then say something like:



That way, you cannot save a new department, and your many to one link would be saved together with the employee.

To answer your question:

No, you can use many to one also to a table where you don't store new data but only retrieve it.

Thanks, it worked and regarding this many-to-one mapping, its for retrieving the objects, right?
When I use Query q = session.createQuery("from Employee"); I get an Employee object.
If I need specific fields, should the query be of this form;
Query q = session.createQuery("select firstName, lastName from Employee");

And how to retrieve specific fields from both the objects? Is it this way:

Query q = session.createQuery("select e.firstName, e.lastName, d.departmentName from Employee e, Department d where e.departmentId=d.departmentId");

Or is there any sophisticated way?

Regards,
Viswanath

You can choose how you select your properties
http://docs.jboss.org/hibernate/stable/core/reference/en/html/queryhql.html#queryhql-select

I would do something like

Or, more readable:
select e.firstName, e.lastName, d.departmentName
from Employee e
inner join employee.department as d

I don't know whether the join in the where clause works in HQL. And especially, you should then be doing where e.department.id as department has a getId() method. Your employee shouldn't have a getDepartmentId() method but a getDepartment() method which returns a Department instance.

The more sophisticated way is returning a map of aliasses and select values, like this:

Or, more readable:
select new map(e.firstName as firstName, e.lastName as lastName, d.departmentName as department)
from Employee e
inner join employee.department as d

Or just do "from Employee" and do the necessary getters (if you do getDepartment() he will join automatically or execute a "select ... from department where department_id = ...." query with the department_id found in the employee.