One-to-one relationships and number of generated SELECTs

mengland · **Joined:** Mon Jun 28, 2004 12:47 pm **Posts:** 2

Hello Hibernaters,

I've got a newbie question about one-to-one mappings. Specfically, about the SQL that Hibernate generates when retrieving rows from tables with one-to-one relationships.

Let's say that I have two tables: A and B. They have a one-to-one relationships. Table B has a FK to A. So the mappings are:

Code:

<class name="example.A" table="A" lazy="false">
    <id name="objId" type="long" column="objId">
        <generator class="increment"/>
    </id>
    <property name="text" column="A_Text" type="string"/>
    <one-to-one name="b" class="example.B" property-ref="a"/>   
</class>

<class name="example.B" table="B" lazy="false">
    <id name="objId" type="long" column="objId">
        <generator class="increment"/>
    </id>
    <property name="text" column="B_Text" type="string"/>
    <many-to-one name="a" class="example.A" column="A_ObjId" unique="true"/>
</class>

Now when I query for rows from A, I want to get the corresponding rows from B as well. I would expect to get a single SQL select statement, like:

Code:

select <cols from A and cols from B> from A join B on B.A_ObjId=A.ObjId

Instead, I get 1+N select statements:

Code:

select <cols from A> from A join B on B.A_ObjId=A.ObjId
select <cols from B> from B where B.A_ObjId=?
.
.
.
select <cols from B> from B where B.A_ObjId=?

Notice that I've used (or tried to use) the "unique foreign key associations" mapping approach suggested in the manual. When I switch to using the "primary key associations" approach, I do get a single select statement.

Now for my question..... is it possible to get a single select statement when using the "foreign key associations" approach to mapping one-to-one relationships?

Thanks,

Mike

PS: there is a post that deals with the difference between primary key associations and unique FK associations. Here: http://forum.hibernate.org/viewtopic.php?t=928211. This post, however, deals with lazy instantiation, rather than join behaviour.

anthony · **Posted:** Mon Jun 28, 2004 5:29 pm

take a look at FETCH

from A as a fetch join a.b should work

mengland · **Joined:** Mon Jun 28, 2004 12:47 pm **Posts:** 2

Anthony,

I thought that FETCH would work, too. But it does not seem to help. Here's the HQL that I'm using:

Code:

Query q = session.createQuery("select a from A as a join fetch a.b");

(Note that this is not identical to your example.... but the snippet that you gave resulted in an error.)

So.... we're still stuck. Other thoughts?

Mike