Re: Ternary associations

[Thomas Mahler]

Hi again,

I followed your advise and added some more details. I thinks it's now 
really a useful example.
See here: 
http://cvs.apache.org/viewcvs/db-ojb/xdocs/faq.xml?rev=HEAD&content-type=text/vnd.viewcvs-markup

anything missing ?

thanks,
Thomas
Charles Anthony wrote:
Hi,

If you think it's useful, feel free.

As Phil has pointed out, the field-descriptors should be marked as PKs. If
you want to flesh it out a little (maybe chuck in the projects class
descriptor, or at the least the collection-descriptor), or change it any way
- be my guest.
Cheers,

Charles.


-----Original Message-----
From: Thomas Mahler [mailto:[EMAIL PROTECTED]
Sent: 04 March 2003 19:26
To: OJB Users List
Subject: Re: Ternary associations
Hi Charles,

thanks for the good explanation and sample (pseudo-) code.
I believed I had to write a testcase with an example, but I 
thinks it's 
not longer required as your sample gives the whole picture.

thanks for saving my evening!
Thomas
PS: If you don't mind I'll place this into our FAQ.

Charles Anthony wrote:

Hi,

OK, I think I understand what you want.

Why is this a "special" scenario ?

You can (and we have in our app) defined a class that represents the
relationship. We have mapped that to a table with the 
corresponding ids. We

can then do queries/updates on that relationship as with any other.

Here's an example : it's worked entirely in the email and is 
probably too

brief, so it doesn't compile and probably has errors. 
However, it should

illustrate that it is possible with OJB.

java class :

public class ProjectRelationship {
 Integer developerId;
 Integer languageId;
 Integer projectId;
 
 Developer developer;  
 Language lanuage;
 Project project;
/** Normal setters and getters **/
}

extract from the repository :

<class-descriptor class="ProjectRelationship" 
table="PROJECTRELATIONSHIP">

 <field-descriptor id="1" name="developerId" column="DEVELOPER_ID"
jdbc-type="INTEGER" />
 <field-descriptor id="2" name="languageId" column="LANGUAGE_ID"
jdbc-type="INTEGER" />
 <field-descriptor id="3" name="projectId" column="PROJECT_ID"
jdbc-type="INTEGER" />
 <reference-descriptor name="developer" class-ref="Developer">
   <foreignkey field-id-ref="1" />
 </reference-descriptor>
 <reference-descriptor name="language" class-ref="Language">
   <foreignkey field-id-ref="2" />
 </reference-descriptor>
 <reference-descriptor name="project" class-ref="Project">
   <foreignkey field-id-ref="3" />
 </reference-descriptor>
</class-descriptor>
storing a relationship :

Developer d = .... (create or retrieve);
Project p = .... (create or retrieve);
Language l = .... (create or retrieve);
ProjectRelationship r = new ProjectRelationship();
r.setDeveloper(dev);
r.setLanguage(l);
r.setProject(p);
Broker.store(r);

querying for all relationships that developer 'bob' has in 'java' :
Criteria criteria = new Criteria();
criteria.addEqualTo("developer.name","bob");
cirteria.addEquatTo("language.name","java");
Query q = new QueryByCriteria(ProjectRelationship.class, 
criteria, true);

Collection relationships = Broker.getCollectionByQuery(q);

You can then iterate through the relationships to get the projects.

You could also have on the Project class-descriptor a
<collection-descriptor> 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".

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);
Hope that helps ,

Cheers,

Charles.



-----Original Message-----
From: O'Reilly John [mailto:[EMAIL PROTECTED]
Sent: 04 March 2003 16:22
To: 'OJB Users List'
Subject: RE: Ternary associations
Hi,

With a ternary relationship there are three (or more) objects 
'related' to
each other.  An example would be 'developer', 'language', 
'project'.  You
want to represent the combinations of these that exist.  It's 
realised by a
bridge table with 3 columns representing the foreign keys in 
the other 3
tables (just like an m:n association is represented by a table with 2
columns).

The following explains how hibernate supports ternary associations.
http://www.xylax.net/hibernate/ternary.html
Regards,
John
-----Original Message-----
From: Charles Anthony [mailto:[EMAIL PROTECTED]
Sent: 04 March 2003 16:08
To: 'OJB Users List'
Subject: RE: Ternary associations
I didn't know exactly what a Ternary association is. 
A quick google suggests that it is what OJB calls an M:N 

association (a

resolution for a many-many relationship).

If it is one of them, then yes, OJB does support it.

See 
http://db.apache.org/ojb/tutorial3.html#mapping%20m:n%20associations

Cheers,

Charles.



-----Original Message-----
From: O'Reilly John [mailto:[EMAIL PROTECTED]
Sent: 04 March 2003 15:58
To: 'OJB Users List'
Subject: RE: Ternary associations
Hi,

Sorry to repost the following question but can someone give 
me a yes/no


answer on this.  If no, then we need to look for another way 
of doing this.

Thanks,
John
-----Original Message-----
From: O'Reilly John [mailto:[EMAIL PROTECTED]
Sent: 26 February 2003 16:59
To: 'OJB Users List'
Subject: Ternary associations
Hi,

According to the following, OJB is supposed to support ternary 
associations:
http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison

I can't find any info on this.  Is it actually supported?

Thanks,
John
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