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From t...@apache.org
Subject cvs commit: db-ojb/xdocs faq.xml
Date Wed, 05 Mar 2003 21:22:08 GMT
thma        2003/03/05 13:22:08

  Modified:    xdocs    faq.xml
  Log:
  add Charles Anthonies example for ternary relationships
  
  Revision  Changes    Path
  1.15      +178 -0    db-ojb/xdocs/faq.xml
  
  Index: faq.xml
  ===================================================================
  RCS file: /home/cvs/db-ojb/xdocs/faq.xml,v
  retrieving revision 1.14
  retrieving revision 1.15
  diff -u -r1.14 -r1.15
  --- faq.xml	6 Feb 2003 01:25:18 -0000	1.14
  +++ faq.xml	5 Mar 2003 21:22:08 -0000	1.15
  @@ -63,6 +63,7 @@
       <li><a href="#28">Global metadata changes at runtime?</a></li>
       <li><a href="#29">Per thread metadata changes at runtime?</a></li>
       <li><a href="#30">Is it possible to use OJB within EJB's?</a></li>
  +	<li><a href="#36">Can OJB handle ternary (or higher) associations?</a></li>
   
      </ul>
   
  @@ -921,6 +922,183 @@
       Additional you can find some EJB example beans in package <code>org.apache.ojb.ejb</code>
       under <code>[jakarta-ojb]/src/ejb</code>.
       </p>
  +    </subsection>
  +
  +    <subsection name="Can OJB handle ternary (or higher) associations?" anchor="36">
  +    <p>
  +    	Yes, that's possible. Here is an example.
  +		With a ternary relationship there are three (or more) entities 
  +		'related' to each other.  
  +		An example would be <code>Developer</code>, <code>Language</code>
and <code>Project</code>.
  +	</p>
  +	<p>	
  +		Each entity is mapped to one table (<code>DEVELOPER</code>, <code>LANGUAGE</code>
and 
  +		<code>PROJECT</code>). To represent the combinations 
  +		of these entities we need an additional bridge table 
  +		(<code>PROJECTRELATIONSHIP</code>)with three 
  +		columns holding the foreign keys to 
  +		the other three	tables (just like an m:n association is represented 
  +		by an intermediary table with 2 columns).
  +	</p>
  +	<p>	
  +		To handle this table with OJB we have to define a class that is mapped on it.
  +		This Relationship class can then be used to perform
  +		queries/updates as with any other persistent class. Here is the layout
  +		of this class:		
  +	</p>		
  +	    <source><![CDATA[
  +public class ProjectRelationship {
  +  Integer developerId;
  +  Integer languageId;
  +  Integer projectId;
  +  
  +  Developer developer;  
  +  Language lanuage;
  +  Project project;
  +  
  +  /** setters and getters not shown for brevity**/
  +}
  +	    ]]></source>
  +	<p>	
  +		Here is the respective extract from the repository :
  +	</p>
  +	    <source><![CDATA[
  +<class-descriptor 
  +	class="ProjectRelationship" 
  +	table="PROJECTRELATIONSHIP"
  +>
  +	<field-descriptor 
  +		name="developerId" 
  +		column="DEVELOPER_ID"
  +		jdbc-type="INTEGER"
  +		primarykey="true"
  +	/>
  +  	<field-descriptor 
  +  		name="languageId" 
  +  		column="LANGUAGE_ID"
  +		jdbc-type="INTEGER" 
  +		primarykey="true"
  +	/>
  +  	<field-descriptor 
  +  		name="projectId" 
  +  		column="PROJECT_ID"
  +		jdbc-type="INTEGER" 
  +		primarykey="true"
  +	/>
  +  	<reference-descriptor 
  +  		name="developer" 
  +  		class-ref="Developer"
  +  	>
  +    	<foreignkey field-id-ref="developerId" />
  +  	</reference-descriptor>
  +  	<reference-descriptor 
  +  		name="language" 
  +  		class-ref="Language"
  +  	>
  +    	<foreignkey field-id-ref="languageId" />
  +  	</reference-descriptor>
  +  	<reference-descriptor 
  +  		name="project" 
  +  		class-ref="Project"
  +  	>
  +    	<foreignkey field-ref="projectId" />
  +  	</reference-descriptor>
  +</class-descriptor>
  +	    ]]></source>
  +
  +<p>
  +Here is some sample code for storing a relationship :
  +</p>
  +<source><![CDATA[
  +Developer dev = .... ; // create or retrieve
  +Project  proj = .... ; // create or retrieve 
  +Language lang = .... ; // create or retrieve
  +
  +ProjectRelationship rel = new ProjectRelationship();
  +rel.setDeveloper(dev);
  +rel.setLanguage(lang);
  +rel.setProject(proj);
  +
  +broker.store(r);
  +]]></source>
  +
  +<p>
  +In the next code sample we are looking up all
  +Projects that Developer "Bob" has done in "Java".
  +</p>
  +<source><![CDATA[
  +Criteria criteria = new Criteria();
  +criteria.addEqualTo("developer.name","Bob");
  +cirteria.addEquatTo("language.name","Java");
  +
  +Query q = new QueryByCriteria(ProjectRelationship.class, criteria, true);
  +Iterator iter = Broker.getIteratorByQuery(q);
  +
  +// now iterate over the collection and retrieve all projects:
  +while (iter.hasNext())
  +{
  +	ProjectRelationship rel = (ProjectRelationship)	iter.next();
  +	System.out.println(rel.getProject().toString());
  +}
  +]]></source>
  +
  +<p>
  +You could also have on the Project class-descriptor a
  +<code>collection-descriptor</code> that returns all relationships associated
with the
  +Project. If it was call "projectRelationships" the following would give you
  +all projects that have a relationship with "bob" and the language "java".
  +</p>
  +
  +<source><![CDATA[
  +Criteria criteria = new Criteria();
  +criteria.addEqualTo("projectRelationships.developer.name","bob");
  +cirteria.addEquatTo("projectRelationships.language.name","java");
  +
  +Query q = new QueryByCriteria(Project.class, criteria, true);
  +Collection projects = Broker.getCollectionByQuery(q);
  +]]></source>
  +
  +<p>
  +This is the layout of the Project class:
  +</p>
  +<source><![CDATA[
  +public class Project {
  +  Integer id;
  +  String name;
  +  Collection projectRelationships;
  +  
  +  /** setters and getters not shown for brevity**/
  +}
  +]]></source>
  +
  +<p>
  +This is the class-descriptor of the Project class:
  +</p>
  +<source><![CDATA[
  +<class-descriptor 
  +	class="Project" 
  +	table="PROJECT"
  +>
  +	<field-descriptor 
  +		name="id" 
  +		column="ID"
  +		jdbc-type="INTEGER"
  +		primarykey="true"
  +	/>
  +  	<field-descriptor 
  +  		name="name" 
  +  		column="NAME"
  +		jdbc-type="VARCHAR" 
  +	/>
  +  	<collection-descriptor 
  +  		name="projectRelationships" 
  +  		element-class-ref="ProjectRelationship"
  +  	>
  +        <inverse-foreignkey field-ref="projectId" />
  +  	</collection-descriptor>  	
  +</class-descriptor>
  +]]></source>
  +
       </subsection>
   
   
  
  
  

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