Hibernate: A Developer's Notebook

5.2. Ordered Collections

Oh, right, that's what we were going to try…

Our first goal is to store the tracks that make up an album, keeping them in the right order. Later we'll add information like the disc on which a track is found, and its position on that disc, so we can gracefully handle multi-disc albums.

5.2.1. How do I do that?

The task of keeping a collection in a particular order is actually straightforward. If that's all we cared about in organizing album tracks, we'd need only tell Hibernate to map a List or array. In our Album mapping we'd use something like Example 5-2.

Example 5-2. Simple ordered mapping of tracks for an album

<list name="tracks" table="ALBUM_TRACKS"> <key column="ALBUM_ID"/> <index column="POSITION"/> <many-to-many class="com.oreilly.hh.Track" column="TRACK_ID"/> </list>

This is very much like the set mappings we've used so far (although it uses a different tag to indicate it's an ordered list and therefore maps to a java.util.List). But notice that we also need to add an index tag to establish the ordering of the list, and we need to add a column to hold the value controlling the ordering in the database. Hibernate will manage the contents of this column for us, and use it to ensure that when we get the list out of the database in the future, its contents will be in the same order in which we stored them. The column is created as an integer, and if possible, it is used as part of a composite key for the table. The mapping in Example 5-2, when used to ...