one-to-one/many-to-one fetch="join" not working.

Marcos Silva Pereira · **Posted:** Fri Jun 02, 2006 7:00 pm

Hibernate version: 3.1.3

Name and version of the database you are using: Postgres 8.1

Hello, I'm having a strange problem with one-to-one relationship. I having the following mappings:
People.hbm.xml:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
   
<hibernate-mapping>
   
   <class name="com.project.model.People" batch-size="5">
   
      <id name="id" unsaved-value="null">
         <generator class="native"/>
      </id>
      
      <property name="nome" />
      <property name="idade" />
      
   </class>
   
</hibernate-mapping>

Group.hbm.xml:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
   
<hibernate-mapping>
   
   <class name="com.project.model.Group" table="tgroup">
   
      <id name="id" unsaved-value="null">
         <generator class="native"/>
      </id>
      
      <property name="description" />
      
      <set name="roles" batch-size="5">
         <key column="id_group" />
         <many-to-many class="com.project.model.Role" column="id_role" />
      </set>
      
   </class>
   
</hibernate-mapping>

User.hbm.xml:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
   
<hibernate-mapping>
   
   <class name="com.project.model.User" table="tuser">
   
      <id name="id" unsaved-value="null">
         <generator class="foreign">
            <param name="property">people</param>
         </generator>
      </id>
      
      <one-to-one name="people" fetch="join" />
      <many-to-one name="group" fetch="join" column="tgroup" />
      
      <property name="login" />
      <property name="password" />
      
   </class>
   
</hibernate-mapping>

When I execute a query like "from User user where user.id = 1, hibernate executes the sql below:

Code:

Hibernate: select user0_.id as id4_, user0_.tgroup as tgroup4_, user0_.login as login4_, user0_.password as password4_ from tuser user0_ where user0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?

But, with fetch="join" set at people and group relationships, the two last queries must not be executed, am I right? I have tried to set hibernate.max_fetch_depth to "2", but this doesn't work too. Anyway, when I retrieve User objects using join fetch in a HQL, all works fine:

Code:

String query = "from User user join fetch user.people people where user.id=5";
Session session = (Session) persistenceSession;
            
return session.createQuery(query).list();

This code generates only this SQL:

Code:

Hibernate: select user0_.id as id4_0_, people1_.id as id2_1_, user0_.tgroup as tgroup4_0_, user0_.login as login4_0_, user0_.password as password4_0_, people1_.nome as nome2_1_, people1_.idade as idade2_1_ from tuser user0_ inner join People people1_ on user0_.id=people1_.id where user0_.id=5

If I put a join fetch to group, this will works properly too. So, what is wrong with my mappings/configurations? Why the fetch="join" attributes are not working?

Kind Regards,

pepelnm · **Posted:** Sun Jun 04, 2006 6:33 am

try with:

Code:

<one-to-one name="people" fetch="join" lazy="false" />
<many-to-one name="group" fetch="join" column="tgroup" lazy="false" /> 

What's happened?

Marcos Silva Pereira · **Posted:** Mon Jun 05, 2006 4:30 pm

pepelnm wrote:

try with:

Code:

<one-to-one name="people" fetch="join" lazy="false" />
<many-to-one name="group" fetch="join" column="tgroup" lazy="false" /> 

What's happened?

I get the same:

Code:

Hibernate: select user0_.id as id4_, user0_.tgroup as tgroup4_, user0_.login as login4_, user0_.password as password4_ from tuser user0_ where user0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?

I think that this configuration should be redundant, because, if I have fetch join, lazy should not be true in any case. Anyway, this configuration seems to be in favor of lazy="false". If I execute the following query without lazy="false" the SQL is like I show below:
HQL: from User user where user.id > 5 // I have 5 registries that matchs with it

Code:

Hibernate: select user0_.id as id4_, user0_.tgroup as tgroup4_, user0_.login as login4_, user0_.password as password4_ from tuser user0_ where user0_.id>?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?

A note about it, groups are clearly loaded in lazy mode. If I not made access to a group, it is not load automatically in join as expected. So, if I put lazy="false", the SQL generated is:

Code:

Hibernate: select user0_.id as id4_, user0_.tgroup as tgroup4_, user0_.login as login4_, user0_.password as password4_ from tuser user0_ where user0_.id>?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?
Hibernate: select people0_.id as id2_0_, people0_.nome as nome2_0_, people0_.idade as idade2_0_ from People people0_ where people0_.id=?
Hibernate: select group0_.id as id0_0_, group0_.description as descript2_0_0_ from tgroup group0_ where group0_.id=?

As I said before, seems that lazy="false" override fetch="join". So, the question is, how to configure mappings to load one-to-one and many-to-one in join mode?

Kind Regards,

scarface · **Joined:** Sun Jun 04, 2006 1:58 am **Posts:** 136

Hibernate is able to fetch only one collection per SQL select , so it isnt possible to declare multiple collections belonging to same persistent calls with fetch="join"

Marcos Silva Pereira · **Posted:** Mon Jun 05, 2006 9:35 pm

scarface wrote:

Hibernate is able to fetch only one collection per SQL select , so it isnt possible to declare multiple collections belonging to same persistent calls with fetch="join"

Ok, I know that, but this case is not about collections, is about one-to-one and many-to-one relationships. I am not at work at now, but I suspect that Hibernate will load group and people (this is a wrong name, it must be Person) only when I use load/get methods. I will try and report it later.

Kind Regards,

Marcos Silva Pereira · **Posted:** Wed Jun 07, 2006 1:00 pm

Well, as I suspect before, only load/get do join fetch:

Code:

Session session = getSession();
User user = (User)session.get(User.class, new Long(1));

This produces the following SQL:

Code:

select 
   user0_.id as id4_2_, 
   user0_.tgroup as tgroup4_2_, 
   user0_.login as login4_2_, 
   user0_.password as password4_2_, 
   people1_.id as id2_0_, 
   people1_.nome as nome2_0_, 
   people1_.idade as idade2_0_, 
   group2_.id as id0_1_, 
   group2_.description as descript2_0_1_ 
from 
   tuser user0_ 
left outer join 
   People people1_ 
on 
   user0_.id=people1_.id 
left outer join 
   tgroup group2_ 
on 
   user0_.tgroup=group2_.id 
where 
   user0_.id=?

Kind Regards,

jbosseur · **Posted:** Wed Jun 07, 2006 3:29 pm

Take a look at the follwing in the Hibernate documentation:

19.1.2. Tuning fetch strategies

Here it says:

Quote:

The fetch strategy defined in the mapping document affects:

retrieval via get() or load()

retrieval that happens implicitly when an association is navigated

Criteria queries

HQL queries if subselect fetching is used

No matter what fetching strategy you use, the defined non-lazy graph is guaranteed to be loaded into memory. Note that this might result in several immediate selects being used to execute a particular HQL query.

Usually, we don't use the mapping document to customize fetching. Instead, we keep the default behavior, and override it for a particular transaction, using left join fetch in HQL. This tells Hibernate to fetch the association eagerly in the first select, using an outer join. In the Criteria query API, you would use setFetchMode(FetchMode.JOIN).

If you ever feel like you wish you could change the fetching strategy used by get() or load(), simply use a Criteria query, for example:

Seems to me that what is said here is exactly what you are expriencing.

Good luck