Fetched Groups - CMR

Question

I'm having problems with CMR fields.
My ejb-jar.xml & sun-ejb-jar.xml files are correct.
Can anyone explain me the use of fetched groups ?
Explanation of the tag <fetched-with></fetched-with>?
Thankx
D.Geerts
The following errors occurs all the time :
Validation error in bean PublicatieE: The field tdUitgeverNId has an invalid fetch group. Default is not a valid fetch group for managed fields. If your field is not a managed field, it may be treated as one if you have a CMR to this bean which is mapped to the same column as tdUitgeverNId.
Select a different fetch group.
[28/Oct/2003:15:38:35] WARNING ( 748): DPL5035:Error while running ejbc
com.iplanet.ias.deployment.backend.IASDeploymentException: Fatal Error from EJB Compiler -- -- Error while processing CMP beans.
     at com.iplanet.ias.deployment.backend.EJBCompiler.wrapException(EJBCompiler.java:589)
     at com.iplanet.ias.deployment.backend.EJBCompiler.compile(EJBCompiler.java:186)
     at com.iplanet.ias.deployment.backend.EjbModuleDeployer.runEJBC(EjbModuleDeployer.java:246)
     at com.iplanet.ias.deployment.backend.EjbModuleDeployer.deploy(EjbModuleDeployer.java:128)
     at com.iplanet.ias.deployment.backend.ModuleDeployer.redeploy(ModuleDeployer.java:581)
     at com.iplanet.ias.deployment.backend.ModuleDeployer.doRequest(ModuleDeployer.java:84)
     at com.iplanet.ias.admin.server.core.mbean.config.ManagedServerInstance.deployEJBJarModuleArchiveOrDirectory(ManagedServerInstance.java:894)
     at com.iplanet.ias.admin.server.core.mbean.config.ManagedServerInstance.deployEJBJarModule(ManagedServerInstance.java:844)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

Answer 1

A fetch group controls when an attribute is fetched. All attributes in the default fetch group are read from the database when the bean is accessed, but attributes in named fetch groups are read only when explicitly accessed. When that happens, all attributes in the same group are read together. Now the problem with managed fields is that you probably have an attribute associated with a column that is also used as a foreign key in a relationship. It then becomes a managed field. What I do in those situations, to get around the problem, is to use a named fetch group, such as "FK" (foreign key) for such attributes.
This is a problem only with database designs that doesn't use synthetic keys, and you therefore need an accessor for the key value itself. With syntheric keys, you would probably get the related bean instead of its key, and you would then not have any attributes mapped to the key column, only relationships.
Hope this was not entirely unclear.