Hibernate: A Developer's Notebook

4.1. Mapping Collections

In any real application you'll be managing lists and groups of things. Java provides a healthy and useful set of library classes to help with this: the Collections utilities. Hibernate provides natural ways for mapping database relationships onto Collections, which are usually very convenient. You do need to be aware of a couple semantic mismatches, generally minor. The biggest is the fact that Collections don't provide "bag" semantics, which might frustrate some experienced database designers. This gap isn't Hibernate's fault, and it even makes some effort to work around the issue.

Bags are like sets, except that the same value can appear more than once.

Enough abstraction! The Hibernate reference manual does a good job of discussing the whole bag issue, so let's leave it and look at a working example of mapping a collection where the relational and Java models fit nicely. It might seem natural to build on the Track examples from Chapter 3 and group them into albums, but that's not the simplest place to start, because organizing an album involves tracking additional information, like the disc on which the track is found (for multi-disc albums), and other such finicky details. So let's add artist information to our database.

As usual, the examples assume you followed the steps in the previous chapters. If not, download the example source as a starting point.

The information we need to keep track of for artists is, at least initially, pretty simple. We'll start ...