Projection

[martinganserer]

Hello,

how is projection implemented in the current release of JBOSS EJB3?

In a new project we want to implement a generic report engine. From my point of view only a good projection implementation can do the job.

Example:

Object[][] result = em.createQuery("select id,articleNo from Article order by articleNo").getResultList();

Would this be possible? Or is it a bad approach?

[bill.burke]

A List of Object[] is returned. Also, take a look at the constructor expressions. you could do this:

public class ArticleSummary {
 public int id;
 public String articleNo;

 public ArticleSummary(int id, String article) {...}
...
}

createQuery("select new com.acme.ArticleSummary(a.id, a.articleNo) From Article a");

You would get back a list of ArticleSummary objects. EJB QL allocates ArticleSummary calling the constructor with the column parameters you queried.

[martinganserer]

Hi Bill,

thank you for your response!
At the moment I am reading a book about Hibernate. In one chapter the book describes projection (in Hibernate) very well. In one example I have seen that Hibernate was able to get a result list without a "hard defined" object like ArticleSummary.
What I want to get is a list containing properties only, as I want to build a generic reporting tool. Generic in this case means that one session bean can create different result lists for different queries. The whole EJB-QL should be created automatically and should return only a data list (like a simple table) with a fixed number of columns and a specific number of rows.

This isn't possible in EJB3, is it?