3.4. Using the Finder
Prev | Chapter 3. Reverse Mapping Tool Tutorial | Next

## 3.4. Using the Finder

The `reversetutorial.Finder` class lets you run queries in JDOQL (JDO's Java-centric query syntax) against the existing database:

    java reversetutorial.Finder <jdoql-query>

JDOQL is discussed in Chapter 11, Query of the JDO Overview. JDOQL looks exactly like Java boolean expressions. To find magazines matching a set of criteria in JDOQL, just specify conditions on the `reversetutorial.Magazine` class' persistent fields. Some examples of valid JDOQL queries for magazines include:

    java reversetutorial.Finder "true"  // use this to list all the magazines
    java reversetutorial.Finder "price < 5.0"
    java reversetutorial.Finder "name == \"Vogue\" || issue > 1000"
    java reversetutorial.Finder "name.startsWith (\"V\")"

To traverse object relations, just use Java's dot syntax:

    java reversetutorial.Finder "publisher.name == \"Adventure\" || publisher.revenue > 1000000"

To traverse collection relations, you have to use JDOQL variables. Variables are just placeholders for any member of a collection. For example, in the following query, `art` is a variable:

    java reversetutorial.Finder "articles.contains (art) && art.title.startsWith (\"JDO\")"

The above query is equivalent to "find all magazines that have an article whose title starts with 'JDO'". The `reversetutorial.Finder` pre-defines two variables you can use: `Article art; Magazine mag;`. With these, you can create very complex queries. For example, to find all magazines whose publisher published an article about "Surpassing Hubble" in any of its magazines:

    java reversetutorial.Finder "publisher.magazines.contains (mag) && mag.articles.contains (art) && art.articleSubtitles.contains (\"Surpassing Hubble\")"

Have fun experimenting with additional queries.

| Note
If you plan on using Kodo JDO to modify data in a schema with non-deferred foreign key constraints, make sure to add the following line to `kodo.properties`:
    kodo.jdbc.ForeignKeyConstraints: true
This line tells Kodo JDO to order its SQL to meet the foreign key constraints of your schema. Note that though this tutorial uses foreign keys with the included Hypersonic database, SolarMetric recommends that you do not use foreign keys with Hypersonic in general use. In fact, in their default configurations the Kodo tools will refuse to create foreign keys in a Hypersonic database. This is because foreign keys seem to destabalize the database under heavy load. Also, make sure to erase the `kodo.jdbc.MappingFactory` setting in `kodo.properties` before continuing with your own JDO experimentation, unless you'd like to always store your mapping information in your JDO metadata files rather than the database.

Prev | Up | Next
3.3. Generating Persistent Classes | Home | Chapter 4. J2EE Tutorial