12.8.3.1. Relationships

Relationships are a special case in lazy loading because a CMR field is both a field and query. As a field it can be on-load block loaded, meaning the value of the currently sought entity and the values of the CMR field for the next several entities are loaded. As a query, the field values of the related entity can be preloaded using on-find.

Again, the easiest way to investigate the loading is to look at a usage scenario. In this example, an HTML table is generated containing each gangster and their hangout. The example code follows:

public String createGangsterHangoutHtmlTable() 
    throws FinderException
{
    StringBuffer table = new StringBuffer();
    table.append("<table>");
    Collection gangsters = gangsterHome.findAll_onfind();
    for (Iterator iter = gangsters.iterator(); iter.hasNext(); ) {
        Gangster gangster = (Gangster)iter.next();

        Location hangout = gangster.getHangout();
        table.append("<tr>");
        table.append("<td>").append(gangster.getName());
        table.append("</td>");
        table.append("<td>").append(gangster.getNickName());
        table.append("</td>");
        table.append("<td>").append(gangster.getBadness());
        table.append("</td>");
        table.append("<td>").append(hangout.getCity());
        table.append("</td>");
        table.append("<td>").append(hangout.getState());
        table.append("</td>");
        table.append("<td>").append(hangout.getZipCode());
        table.append("</td>");
        table.append("</tr>");
    }

    table.append("</table>");return table.toString();
}

Example 12.3. Relationship Lazy Loading Example Code

For this example, the configuration of the gangster's findAll_onfind query is unchanged from the on-find section. The configuration of the Location entity and Gangster-Hangout relationship follows:

<jbosscmp-jdbc>
    <enterprise-beans>
        <entity>
            <ejb-name>LocationEJB</ejb-name>
            <load-groups>
                <load-group>
                    <load-group-name>quick info</load-group-name>
                    <field-name>city</field-name>
                    <field-name>state</field-name>
                    <field-name>zipCode</field-name>
                </load-group>
            </load-groups>
            <eager-load-group/>
        </entity>
    </enterprise-beans>
    <relationships>
        <ejb-relation>
            <ejb-relation-name>Gangster-Hangout</ejb-relation-name>
            <foreign-key-mapping/>
            <ejb-relationship-role>
                <ejb-relationship-role-name>
                    gangster-has-a-hangout
                </ejb-relationship-role-name>
                <key-fields/>
                <read-ahead>
                    <strategy>on-find</strategy>
                    <page-size>4</page-size>
                    <eager-load-group>quick info</eager-load-group>
                </read-ahead>
            </ejb-relationship-role>
            <ejb-relationship-role>
                <ejb-relationship-role-name>
                    hangout-for-a-gangster
                </ejb-relationship-role-name>
                <key-fields>
                    <key-field>
                        <field-name>locationID</field-name>
                        <column-name>hangout</column-name>
                    </key-field>
                </key-filaelds>
            </ejb-relationship-role>
        </ejb-relation>
    </relationships>
</jbosscmp-jdbc>

Example 12.4. The jbosscmp-jdbc.xml Relationship Lazy Loading Configuration

JBoss will execute the following query for the finder:

SELECT t0_g.id, t0_g.name, t0_g.nick_name, t0_g.badness
    FROM gangster t0_g
    ORDER BY t0_g.id ASC

Then when the hangout is accessed, JBoss executes the following two queries to load the city, state, and zip fields of the hangout:

SELECT gangster.id, gangster.hangout,
       location.city, location.st, location.zip
    FROM gangster, location
    WHERE (gangster.hangout=location.id) AND
          ((gangster.id=0) OR (gangster.id=1) OR
          (gangster.id=2) OR (gangster.id=3))
SELECT gangster.id, gangster.hangout,
       location.city, location.st, location.zip
    FROM gangster, location
    WHERE (gangster.hangout=location.id) AND
          ((gangster.id=4) OR (gangster.id=5) OR
          (gangster.id=6) OR (gangster.id=7))

The following table shows the execution of the queries:

id	name	nick_name	badness	hangout	id	city	st	zip
0	Yojimbo	Bodyguard	7	0	0	San Fran	CA	94108
1	Takeshi	Master	10	1	1	San Fran	CA	94133
2	Yuriko	Four finger	4	2	2	San Fran	CA	94133
3	Chow	Killer	9	3	3	San Fran	CA	94133
4	Shogi	Lightning	8	4	4	San Fran	CA	94133
5	Valentino	Pizza-Face	4	5	5	New York	NY	10017
6	Toni	Toothless	2	6	6	Chicago	IL	60661
7	Corleone	Godfather	6	7	7	Las Vegas	NV	89109

Table 12.4. on-find Optimized Relationship Query Execution