Hi,

I have two classes Foo and Bar. Foo may have 0 or one Bar objects. One Bar object can only belong to one Foo object.

Should we use one-to-one association to model this? I tried this using the following:

On Foo class
<one-to-one name="bar"
class="Bar">
</one-to-one>


On Bar class

<one-to-one name="foo" class="Foo" constrained="true"/>

With this mapping, in the database table of Foo, I don't have bar column. Is this expected? When I save foo and bar by doing :

session.save(bar);
session.save(foo);
I get error
Hibernate error not-null property references a null or transient value: bar:foo.

Can someone give me a hint?

Thanks.

-Jing

try using the outer-join="true". This would force Hibernate to do a outerjoin wherein if Foo does not have a Bar it would still return Foo by very nature of Outerjoins.

Hope that helps.

-anand

on second thoughts, try this in the mapping:


constrained property would make hibernate think it's a foreign key constraint and expect that Foo is not-null.

-Anand

Another more cleaner implementation would be to use a bi-directional <many-to-one> relationship. The documentation has a similar example (Page 37 in the PDF), Employer-Person with a bi-directional relationship.

-Anand

I am having a problem with one-to-one relationships in the mapping files and tried to figure it out (i read through the documentation and everything, but I still can't get it).

I am playing with my own bank example. I have User objects and Account objects. Here are my relationships...

1. A User can have many Accounts, but an Account can only belong to one user (many-to-one).
2. A User may have a "preferred" account, but it is not required (this is one-to-one cardinality, but one-to-zero participation).

In my User object I have a reference to the preferred account object (with the appropriate getters and setters). Here is what my User.hbm.xml looks like...



Should the <one-to-one> tag be in the Account.hbm.xml? How should I set up the one-to-one tag?

Thanks!
--
Nathan

Thanks for your quick reply.

I put the outer-join thing in, but I encountered some other problems. Basically, hibernate document suggests that one-to-one have two varieties to link two classes together: use the same primary key and use foreign key. But I cannot use primary key approach to link two class, as it needs to use "foreign" id generator strategy in Foo class. In our case, we have assigned id in Foo class.

So here is what I have:

In Foo class:

<id name="ID" column="fooID" type="long" unsaved-value="null">
<generator class="assigned"/>
</id>

<one-to-one name="bar" class="Bar" outer-join="true"/>


in Bar class:

<id name="bID" column="bID" type="long" unsaved-value="null">
<generator class="native"/>
</id>
<many-to-one name="fooID" class="Foo" column="fooID" unique="true"/>



When I save my Bar object, I get error:
Hibernate error No persister for: java.lang.Long


In my Bar class, bID and fooID are the only two attributes of type Long. Other attributes are type of String.
I have getter and setter for both bID and fooID class.

Thanks.
-Jing

Here's the mapping I used:



so when you load Foo using the following code:



and set the "show.sql" property of hibernate to "true". YOu'll see the following in your log:



If you set the "outer-join" to false, you'll see the log printing 2 select statements 1 for each table.

To save "Foo" with no Bars, you'll have code like this: