Hibernate: A Developer's Notebook

9.2. Selecting Properties and Pieces

The queries we've been using so far have returned entire persistent objects. This is the most common use of an object/relational mapping service like Hibernate, so it should come as no surprise. Once you've got the objects, you can use them in whatever way you need to within the familiar realm of Java code. There are circumstances where you might want only a subset of the properties that make up an object, though, such as producing reports. HQL can accommodate such needs, in exactly the same way you'd use ordinary SQL—projection in a select clause.

9.1.1. How do I do that?

Suppose we want to change QueryTest.java to display only the titles of the tracks that meet our search criteria, and we want to extract only that information from the database in the first place. We'd start by changing the query of Example 3-9 to retrieve only the title property. Edit Track.hbm.xml to make the query look like Example 9-6.

Example 9-6. Obtaining just the titles of the short tracks

<query name="com.oreilly.hh.tracksNoLongerThan"> <![CDATA[ select track.title from com.oreilly.hh.Track as track where track.playTime <= :length ]]> </query>

Make sure the tracksNoLongerThan() method in QueryTest.java is set up to use this query. (If you edited it to use criteria queries in Chapter 8, change it back to the way it was in Example 3-10. To save you the trouble of hunting that down, it's reproduced as Example 9-7.)

Example 9-7. HQL-driven query method, using the query mapped ...