I have an issue with one-to-one mappings, and I am wondering whether the issue I encounter is a NHibernate (1.0.2) bug, or a problem with my understanding of one-to-one's.

The situation I have is quite simple. A person has one account and an account is for one person only.

Database:
Person (Id, Account)
Account(Id)

Mapping file for account:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class lazy="true" name="Account" table="Account">
<id name="Id" type="Guid" unsaved-value="00000000-0000-0000-0000-000000000000">
<column name="Id" sql-type="uniqueidentifier" not-null="true" unique="true" index="PK_Account"/>
<generator class="guid.comb" />
</id>
<one-to-one name="Person" cascade="all" constrained="true" property-ref="Account" class="ResPerson"/>
</class>
</hibernate-mapping>


Mapping file for person:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class lazy="true" name="ResPerson" table="ResPerson">
<id name="Id" type="Guid" unsaved-value="00000000-0000-0000-0000-000000000000">
<column name="Id" sql-type="uniqueidentifier" not-null="true" unique="true" index="PK_ResPerson"/>
<generator class="guid.comb" />
</id>
<many-to-one name="Account" class="Account" cascade="all">
<column name="AccountRef" sql-type="uniqueidentifier" not-null="false" index="IX_Account"/>
</many-to-one>
</class>
</hibernate-mapping>


The issue is, that when I create an account and set the person for an account (Account account = new Account(); account.Person = aPerson) the person table is not updated with the key to Person.

When I create a new account and do it the other way around (Account account = new Account(); aPerson.Account = account) the foreign key is set correctly.

Does anybody have any tips?

Thanks, Bastiaan de Rijber

I think you should use <generator class="foreign"> in one of those classes.

Thanks for your tip Sergey, but from what I gather the foreign generator is used primarily for one-to-one relationsships where you'd use the primary key to to match the two tables - e.g. you use Account.Id to match Person.Id - in this case I have a foreign key on Person. Now, what I might have been missing is a unique constraint on the many-to-one, as mentioned in the documentation, but I fail to see how this would impact hibernate's persistance and/or property set logic.

Ah ok, it was me getting stupid again, sorry... The person table is not updated in the first case because you are not changing the person, only the account.

Essentially, the information contained in Person object gets stored into ResPerson table, and the information in the Account objects gets stored into Account table, according to the mapping files. NHibernate doesn't try to be smart here and leaves it to you to maintain both sides of the relationship.

Ahh, right... so it's basically the same situation as with the one-to-many and many-to-one mappings.

It's not quite what you'd expect for one-to-ones, but I guess if you are aware of what's actually happening there it's understandable.

So, basically, the suggested approach for this kind of situation is to change the domain model?

Just make sure that when you are setting Person.Account you also set Account.Person, and vice versa. This should be enough.