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ctheiss
 Post subject: Why can't I handle one-to-one relationships efficiently?
PostPosted: Fri Oct 12, 2007 2:51 pm 
Newbie

Joined: Fri Oct 12, 2007 12:23 pm
Posts: 9
Hibernate version: 3.2.5.ga

I've managed to find some odd behaviour with one-to-one relationships. I'm hoping this is because I'm not handling the mappings properly, but I basically copied them directly from the documentation.

Here's my tables / objects / hbm files:

Code:
a(id INT AUTO_INCREMENT, bId INT NOT NULL, PRIMARY KEY(id))
b(id INT AUTO_INCREMENT, PRIMARY KEY(id))

class A {
    private int id;
    private B b;
   
    // getters / setters ...
}

class B {
    private int id;
    private A a;
   
    // getters / setters ...
}

<class name="A" table="a">
  <id name="id" column="id" type="java.lang.Long">
    <generator class="native">
      <param name="sequence">global_seq</param>
    </generator>
  </id>

  <many-to-one name="b" column="bId" not-null="true" unique="true" cascade="save-update" />
</class>

<class name="B" table="b">
  <id name="id" column="id" type="java.lang.Long">
    <generator class="native">
      <param name="sequence">global_seq</param>
    </generator>
  </id>

  <one-to-one name="a" class="A" constrained="true" property-ref="b" cascade="save-update" />
</class>

Now, when I get a list of A, my log file shows n+1 selects:
Code:
org.hibernate.SQL - select this_.id as id33_6_, this_.bId as a_33_6_, b_.id as id31_1_ from a this_ inner join b b_ on this_.bId=b_.id
org.hibernate.SQL - select a_.id as id33_0_, a_.bId as a2_33_0_ from a a_ where a_.bId=?
org.hibernate.SQL - select a_.id as id33_0_, a_.bId as a2_33_0_ from a a_ where a_.bId=?
org.hibernate.SQL - select a_.id as id33_0_, a_.bId as a2_33_0_ from a a_ where a_.bId=?
...

The hibernate log suggests that it needs to "initialize the entity" in those extra selects:
Code:
org.hibernate.loader.Loader - result set row: 0
org.hibernate.loader.Loader - result row: EntityKey[B#8117], EntityKey[A#18877]
org.hibernate.loader.Loader - result set row: 1
org.hibernate.loader.Loader - result row: EntityKey[B#8118], EntityKey[A#18878]
org.hibernate.loader.Loader - result set row: 2
org.hibernate.loader.Loader - result row: EntityKey[B#8119], EntityKey[A#18879]
...
org.hibernate.engine.TwoPhaseLoad - resolving associations for [B#8117]
org.hibernate.loader.Loader - loading entity: [A#8117]
org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
org.hibernate.jdbc.AbstractBatcher - about to open ResultSet (open ResultSets: 0, globally: 0)
org.hibernate.loader.Loader - result set row: 0
org.hibernate.loader.Loader - result row: EntityKey[A#18877]
org.hibernate.jdbc.AbstractBatcher - about to close ResultSet (open ResultSets: 1, globally: 1)
org.hibernate.jdbc.AbstractBatcher - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
org.hibernate.loader.Loader - done entity load
org.hibernate.engine.TwoPhaseLoad - done materializing entity [B#8117]
org.hibernate.engine.TwoPhaseLoad - resolving associations for [A#18877]
org.hibernate.engine.TwoPhaseLoad - done materializing entity [A#18877]
org.hibernate.engine.TwoPhaseLoad - resolving associations for [B#8118]
org.hibernate.loader.Loader - loading entity: [A#8118]
org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
org.hibernate.jdbc.AbstractBatcher - about to open ResultSet (open ResultSets: 0, globally: 0)
org.hibernate.loader.Loader - result set row: 0
org.hibernate.loader.Loader - result row: EntityKey[A#18878]
org.hibernate.jdbc.AbstractBatcher - about to close ResultSet (open ResultSets: 1, globally: 1)
org.hibernate.jdbc.AbstractBatcher - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
org.hibernate.loader.Loader - done entity load
org.hibernate.engine.TwoPhaseLoad - done materializing entity [B#8118]
org.hibernate.engine.TwoPhaseLoad - resolving associations for [A#18878]
org.hibernate.engine.TwoPhaseLoad - done materializing entity [A#18878]
org.hibernate.engine.TwoPhaseLoad - resolving associations for [B#8119]
org.hibernate.loader.Loader - loading entity: [A#8119]
org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
org.hibernate.jdbc.AbstractBatcher - about to open ResultSet (open ResultSets: 0, globally: 0)
org.hibernate.loader.Loader - result set row: 0
org.hibernate.loader.Loader - result row: EntityKey[A#18879]
org.hibernate.jdbc.AbstractBatcher - about to close ResultSet (open ResultSets: 1, globally: 1)
org.hibernate.jdbc.AbstractBatcher - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
org.hibernate.loader.Loader - done entity load
org.hibernate.engine.TwoPhaseLoad - done materializing entity [B#8119]
org.hibernate.engine.TwoPhaseLoad - resolving associations for [A#18879]
org.hibernate.engine.TwoPhaseLoad - done materializing entity [A#18879]
...

Any reason why hibernate thinks it must select again for object a, when it could get that information from the original query? I've found that if I add a fetch="join" to the many-to-one relationship, it just performs this select:
Code:
select ... from a inner join b left outer join a

Ideally, I would like to remove any re-querying or joining of table a. Thanks for your help!
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ctheiss
 Post subject:
PostPosted: Fri Oct 12, 2007 3:05 pm 
Newbie

Joined: Fri Oct 12, 2007 12:23 pm
Posts: 9
I've been going through the hibernate code to try to figure out what's going on. It seems like it's trying to populate the field (B.a) from a "unique key cache" on the session. Unfortunately, this cache is empty.

There is one place that the Loader populates this cache: in loadFromResultSet. The following line always seems to return "null" for me:

Code:
String ukName = ownerAssociationTypes[i].getRHSUniqueKeyPropertyName();

However, if I change ukName to "b" using my debugger, everything works!

I know I'm getting a little esoteric here; any ideas whether I'm going down the right path?
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ctheiss
 Post subject:
PostPosted: Mon Oct 15, 2007 9:54 am 
Newbie

Joined: Fri Oct 12, 2007 12:23 pm
Posts: 9
Hmmmm.... no bites.

Well, if anybody else has this problem, I'm afraid you'll find our solution most unsatisfying. We're taking advantage of the fact that whenever we need a list of A, we will not change any of the objects; it is read only.

So, we used a view to merge the tables in SQL, and load them all into a new object called AB.
Corey
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ctheiss
 Post subject:
PostPosted: Mon Oct 15, 2007 9:55 am 
Newbie

Joined: Fri Oct 12, 2007 12:23 pm
Posts: 9
Hmmmm.... no bites.

Well, if anybody else has this problem, I'm afraid you'll find our solution most unsatisfying. We're taking advantage of the fact that whenever we need a list of A, we will not change any of the objects; it is read only.

So, we used a view to merge the tables in SQL, and load them all into a new read-only object called AB.
Corey
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