> d. You say that I should use an 'extra' attribute in class 'CityVO' to
> represent the 'foreign key' in the table. But, in my opinion, this breaks
> with OO design, as I am using 'relational concepts' instead of 'OO
> concepts'. This is one of the problems of the 'impedance mismatch' and I
> think the OO way to solve it is using OO design the way I suggested.
I kind of agree with you here! However OJB supports anonymous fields -
columns that exist in the database, but have no field in the associated
class, and these allow you to do what you are doing.
<field-descriptor
id="3"
name="state::codState"
column="ID_STATE"
jdbc-type="INTEGER"
access="anonymous"/>
> b. You say I don't have to instantiate the variable 'state' inside the
> constructor of class 'CityVO'. But if I don't put that code in the
> constructor I get the following exception:
I dont understand what is causing this. What version of OJB are you using?
Maybe the anonymous field above is causing the problem? Not sure what OJB
does if access="anonymous" is not set, and there is no field in the class.
Also, you might like to change the name, as it isnt a valid java variable
name.
Daniel.
> -----Original Message-----
> From: Enrique Medina [mailto:[EMAIL PROTECTED]
> Sent: 20 May 2004 19:00
> To: [EMAIL PROTECTED]
> Subject: RE: Reference proxies & Interfaces
>
>
> Thanks very much for your comments. Just a pair of comments:
>
> b. You say I don't have to instantiate the variable 'state' inside the
> constructor of class 'CityVO'. But if I don't put that code in the
> constructor I get the following exception:
>
> java.lang.InstantiationException: StateProxy
> at
> org.apache.ojb.broker.metadata.fieldaccess.AbstractPersistentField
> .setNestedObject(AbstractPersistentField.java:329)
> at
> org.apache.ojb.broker.metadata.fieldaccess.AbstractPersistentField
> .set(AbstractPersistentField.java:100)
> at
> org.apache.ojb.broker.accesslayer.RowReaderDefaultImpl.buildWithRe
> flection(RowReaderDefaultImpl.java:260)
> at
> org.apache.ojb.broker.accesslayer.RowReaderDefaultImpl.readObjectF
> rom(RowReaderDefaultImpl.java:77)
> at
> org.apache.ojb.broker.accesslayer.RsIterator.getObjectFromResultSe
> t(RsIterator.java:488)
> at
> org.apache.ojb.broker.accesslayer.RsIterator.next(RsIterator.java:288)
> at
> org.apache.ojb.broker.accesslayer.PagingIterator.next(PagingIterat
> or.java:187)
> at
> org.apache.ojb.broker.core.QueryReferenceBroker.getCollectionByQue
> ry(QueryReferenceBroker.java:171)
> at
> org.apache.ojb.broker.core.QueryReferenceBroker.getCollectionByQue
> ry(QueryReferenceBroker.java:211)
> at
> org.apache.ojb.broker.core.QueryReferenceBroker.getCollectionByQue
> ry(QueryReferenceBroker.java:223)
> at
> org.apache.ojb.broker.core.PersistenceBrokerImpl.getCollectionByQu
> ery(PersistenceBrokerImpl.java:1205)
> at
> org.apache.ojb.broker.core.DelegatingPersistenceBroker.getCollecti
> onByQuery(DelegatingPersistenceBroker.java:331)
> at
> org.apache.ojb.broker.core.DelegatingPersistenceBroker.getCollecti
> onByQuery(DelegatingPersistenceBroker.java:331)
>
> d. You say that I should use an 'extra' attribute in class 'CityVO' to
> represent the 'foreign key' in the table. But, in my opinion, this breaks
> with OO design, as I am using 'relational concepts' instead of 'OO
> concepts'. This is one of the problems of the 'impedance mismatch' and I
> think the OO way to solve it is using OO design the way I suggested.
>
> Regards,
> Enrique Medina.
>
>
> >From: "Daniel Perry" <[EMAIL PROTECTED]>
> >Reply-To: "OJB Users List" <[EMAIL PROTECTED]>
> >To: "OJB Users List" <[EMAIL PROTECTED]>
> >Subject: RE: Reference proxies & Interfaces
> >Date: Thu, 20 May 2004 18:43:37 +0100
> >
> >I ended up spending time trying to understand proxies... confusing stuff
> >when you start out with ojb... these days it's making a little
> more sense
> >:)
> >
> >a. yes, it has to be of type StateProxy (the interface) as ojb
> will try to
> >instanciate it with a proxy object that implements the interface. If you
> >use a StateVO, ojb cant just point it to a proxy.
> >
> >b. I didnt think so. I'm pretty sure my code was not doing this, and was
> >working fine.
> >
> >c. This is correct. In fact you dont need the CityProxy
> interface at all.
> >Early on i got it into my head that i needed interfaces for collection
> >proxies like this. It was only the other day that i realised otherwise!
> >
> >d. dont quite understand the code here... i would use fkStateId and then
> >have "private int fkStateId;" in CityVO +interface.
> >
> >To be honest, i barely ever use reference proxies any more as i
> found that
> >they didnt have much effect on performance of my apps, and i had to use
> >interfaces everywhere. I almost always proxy collections
> though, as these
> >do make a big difference.
> >
> >Daniel.
> >
> > > -----Original Message-----
> > > From: Enrique Medina [mailto:[EMAIL PROTECTED]
> > > Sent: 20 May 2004 18:25
> > > To: [EMAIL PROTECTED]
> > > Subject: Reference proxies & Interfaces
> > >
> > >
> > > This is just a confirmation question about using reference
> > > proxies in OJB. I
> > > have been using them in all my applications, but I would like
> to confirm
> > > that I am making a good use of them.
> > >
> > > So my way of using reference proxies is:
> > >
> > > 1) Let's consider two business entities: state and city. So I have:
> > >
> > > public interface StateProxy
> > > {
> > > public Integer getCodState();
> > > public String getDescState();
> > > public Collection getCities();
> > >
> > > public void setCodState(Integer state);
> > > public void setDescState(String description);
> > > public void setCities(Collection cities);
> > > }
> > >
> > > public class StateVO implements StateProxy, Serializable
> > > {
> > > private Integer codState;
> > > private String descState;
> > >
> > > private Collection cities;
> > >
> > > public StateVO()
> > > {
> > > }
> > >
> > > // Getters/Setters ....
> > > }
> > >
> > > public interface CityProxy
> > > {
> > > public Integer getCodCity();
> > > public String getDescCity();
> > > public StateProxy getState();
> > >
> > > public void setCodCity(Integer city);
> > > public void setDescCity(String description);
> > > public void setState(StateProxy state);
> > > }
> > >
> > > public class CityVO implements CityProxy, Serializable
> > > {
> > > private Integer codCity;
> > > private String descCity;
> > >
> > > private StateProxy state;
> > >
> > > public CityVO()
> > > {
> > > state = new StateVO();
> > > }
> > >
> > > // Getters/Setters ....
> > > }
> > >
> > > 2) Do the mapping in repository-user.xml:
> > >
> > > <class-descriptor
> > > class="StateVO"
> > > table="TBL_STATE"
> > > >
> > >
> > > <field-descriptor
> > > id="1"
> > > name="codState"
> > > column="ID_STATE"
> > > jdbc-type="INTEGER"
> > > primarykey="true" />
> > >
> > > <field-descriptor
> > > id="2"
> > > name="descState"
> > > column="DESC_STATE"
> > > jdbc-type="VARCHAR" />
> > >
> > > <collection-descriptor
> > > name="cities"
> > > element-class-ref="CityVO"
> > > proxy="true">
> > >
> > > <inverse-foreignkey field-id-ref="3" />
> > >
> > > </collection-descriptor>
> > >
> > > </class-descriptor>
> > >
> > > <class-descriptor
> > > class="CityVO"
> > > table="TBL_CITY"
> > > >
> > >
> > > <field-descriptor
> > > id="1"
> > > name="codCity"
> > > column="ID_CITY"
> > > jdbc-type="INTEGER"
> > > primarykey="true" />
> > >
> > > <field-descriptor
> > > id="2"
> > > name="descCity"
> > > column="DESC_CITY"
> > > jdbc-type="VARCHAR" />
> > >
> > > <field-descriptor
> > > id="3"
> > > name="state::codState"
> > > column="ID_STATE"
> > > jdbc-type="INTEGER" />
> > >
> > > <reference-descriptor
> > > name="state"
> > > class-ref="StateVO"
> > > proxy="true">
> > >
> > > <foreignkey field-id-ref="3" />
> > >
> > > </reference-descriptor>
> > >
> > > </class-descriptor>
> > >
> > > And my questions are the following:
> > >
> > > a) I declare the variable 'state' in class 'CityVO' as being of type
> > > 'StateProxy' (the interface). If not, I get errors from OJB
> indicating
> >it
> > > can't obtain a getter/setter for the 'Proxy' object (I suppose it
> > > tries to
> > > obtain a getter/setter for the interface, not for the
> > > implementacion class).
> > > Is this the right behaviour?
> > >
> > > b) I have to instantiate the variable 'state' in class 'CityVO'
> > > inside the
> > > default constructor, because if not, OJB throws an exception saying it
> > > cannot instantiate 'StateProxy'. I suppose this must be correct.
> > >
> > > c) With the collection variable 'cities' in class 'StateVO',
> > > there's no need
> > > to instantiate it on the constructor, as OJB knows how to instantiate
> >any
> > > collection. Am I right?
> > >
> > > d) Do you think is correct to map the 'foreign key' using the variable
> > > 'state' defined in class 'CityVO' in the way 'state::codState' in the
> > > class-descriptor for 'CityVO'?
> > >
> > > I would welcome any suggestions, critics and comments from anyone
> > > that has
> > > worked with reference proxies and interfaces with OJB.
> > >
> > > Thanks in advance,
> > > Enrique Medina.
> > >
> > > _________________________________________________________________
> > > �D�nde se esconden [EMAIL PROTECTED] [EMAIL PROTECTED] Encuentra miles de
> > > perfiles en
> > > MSN Amor &
> > > Amistad. http://match.msn.es/
> > >
> > >
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> > >
> > >
> >
> >
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>
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