I am trying to build a zero-to-one or zero-to-many mapping. However, I have not been able to find any information on this in the documentation, forums, or the web in general.

An example of the relationship I am trying to set up would be a Person record that may or may not have an associated Address record. (I'm working with legacy data, so normalizing to remove the 1-to-1 relationship isn't easy.)

My mappings look like the following:

Person
[code]
<hibernate-mapping schema="KELIB">
<class name="com.test.data.Person" table="Person">
<id name="id" column="ID" type="string">
<generator class="assigned" />
</id>
<property name="name" column="NAME" />
<property name="dob" column="DOB" />
<many-to-one name="address" class="com.test.data.Address" column="id" not-null="true" />
</class>
</hibernate-mapping>
[\code]

Address mapping
[code]
<hibernate-mapping schema="KELIB">
<class name="com.test.data.Address" table="ADDRESS">
<id name="id" column="ID" type="string">
<generator class="assigned" />
</id>
<property name="street" column="STREET" />
<property name="city" column="CITY" />
<property name="state" column="STATE" />
<property name="zip" column="ZIP" />
</class>
</hibernate-mapping>
[/code]

When I run a test that tries to load either a full set of Persons (through session.iterate) or a single Person that does not have an associated Address record, I get the following exception:

UnresolvableObjectException
[code]
net.sf.hibernate.UnresolvableObjectException: No row with the given identifier exists: 100005728, of class: com.test.data.Address
at net.sf.hibernate.UnresolvableObjectException.throwIfNull(UnresolvableObjectException.java:38)
at net.sf.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1881)
at net.sf.hibernate.type.ManyToOneType.resolveIdentifier(ManyToOneType.java:68)
at net.sf.hibernate.type.EntityType.resolveIdentifier(EntityType.java:215)
at net.sf.hibernate.impl.SessionImpl.initializeEntity(SessionImpl.java:2132)
at net.sf.hibernate.loader.Loader.doQuery(Loader.java:239)
at net.sf.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:132)
at net.sf.hibernate.loader.Loader.doList(Loader.java:949)
at net.sf.hibernate.loader.Loader.list(Loader.java:940)
at net.sf.hibernate.hql.QueryTranslator.list(QueryTranslator.java:833)
at net.sf.hibernate.impl.SessionImpl.find(SessionImpl.java:1475)
at net.sf.hibernate.impl.QueryImpl.list(QueryImpl.java:39)
at com.test.test.data.SpecialDataTests.testAdmindLoadsNullForMissingPerson(SpecialDataTests.java:64)
at java.lang.reflect.Method.invoke(Native Method)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:392)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:276)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:167)
[/code]

This UnresolvableObjectException exception is understandable, because that is what is happening. What I can't figure out is how to tell Hibernate that it is OK for a Person to not have an Address.

A few other notes:

- I'm using Hibernate 2.1.1 against a DB2 database.

- I also tried this with a one-to-one mapping, with the same results.

- Catching the exception is not helpful, because it is halting the entire session.iterate process. I don't mind handling the exception, but I do need to continue loading the Person records, including the one with the missing Address.

- I have a workaround of loading each Person and then each Address manually and adding the Address to the Person object, but I would prefer to let Hibernate handle the relationship.

- I have seen mention of using an Interceptor or UserType to solve similar problems, but I can't find documentation on these either.

column="id" is wrong, id should refer to the PK of Adress Table, not Person. Hibernate try to load an address with a person PK (as you requested).
There should be a FK column from person to address.

Look, you specified column="id" in your <many-to-one> mapping. But "id" column is already used for Person id! You must place the name of the column which contains Address, not Person Id.

Thanks, but ...

"id" is the PK of Address in the example above.

I tried giving the Address PK a different name ("AID") and changing the <many-to-one> tag, but then I get the error below, which seems to indicate that putting the PK of Address in the <many-to-one> tag in Person will not work.

Note that Hibernate does load Person objects correctly as long as they have an associated Address. I am trying to figure out how to let Hibernate know that a missing Address is OK and I don't want an UnresolvableObjectException.

[b]Exception given when Address PK is in Person <many-to-one> tag[\b]

1. column="" must not to the PK of ADDRESS table, but to name of the column in PERSON table which is used to hold address id. Do you have such a column or you are expecting addressId==personId?

2. not-null must be "false" if you need to allow Person without an Address. And person without an address is a record from PERSON tabel with ADDRESS_ID set to NULL.

I am expecting addressId==personId.

I switched not-null to false - which is what my running code looks like - I don't know which gremlins changed my sample! :)

I also changed to the column name "ID" instead of the mapping name "id" this caused a MappingException that was solved by adding insert="false" update="false" to the <many-to-one>.

Now the same error occurs - throwing UnresolveableObjectException for a Person without an Address.

I think the second part of your #2 sheds light on the issue. Since the PK of Person is also the linked to Address, and by definition is populated, Hibernate assumes the relationship is not null and tries to load the Address.

Maybe I need to figure out a way to configure the mapping from the other direction. In my data, any Address will have a Person, but not all Persons have an Address. The hard part is that I need to be able to load all Persons with and without Addresses, so I can't just use Address as the master.

Thanks for your help!

You are cheatin here. You clearly have <one-to-one> relationship

Er.. I mean you should consider implementing your relationship using <one-to-one> instead <many-to-one>

Guilty as charged!

I did mention in the original post that I had used <one-to-one> previously with the same effect.

However, I just changed the code back to <one-to-one> and it works.

Argh! I hate programming by accident, as Dave Thomas calls it.

My best guess, for those who follow, is that I previously had a composite-id on Person that I switched to a regular id during this process.

Thanks dimas for your help. I have at least learned more about how the columns are mapped in these relationships!