One-to-one mapping problem

jdenzel · **Joined:** Mon Jan 05, 2004 10:46 am **Posts:** 13

Hi,

I have the following class which is used to build a tree of groups:

Code:

public final class SystemEventGroup
{
    private long id;
    private SystemEventGroup parentEventGroup;
    private String name;
    private String descr;

    // getters and setters
    ...
}

I could not figure out how to write the mapping for the parentEventGroup
property.

How can a mapping for such recursive object graphs be realized? I searched the forum but could not find a solution.

Any help greatly appreciated.

Thanks,

juergen

bertrand · **Posted:** Tue Feb 17, 2004 6:34 am

I'm not sure a one-to-one mapping is what you need: will a parent have possibly more than one child, but a child a single parent ?
In such case, you should use a many-to-one (read from the Child):

Code:

   <class name="SystemEventGroup" >

      <!-- Unique Identifier -->
      <id name="id" type="long">
         <generator ....>
        ....
         </generator>
      </id>
      
      <property name="name" />
      <property name="descr" />
    
      <many-to-one
        name="parentEventGroup"
      />
    
    ....
  </class>

Note: if a one-to-one is what you need, just replace the many-to-one...

jdenzel · **Joined:** Mon Jan 05, 2004 10:46 am **Posts:** 13

I have a 1..1 association. I tried your solution approach, but this does not work.

Here is my mapping:

Code:

    <class name="wilken.openshop.core.event.SystemEventGroup" table="system_event_group">
        <id name="id" type="long" unsaved-value="-1">
            <column name="id" sql-type="int(11)" not-null="true"/>
            <generator class="identity"/>
        </id>
        <one-to-one name="parentEventGroup" class="wilken.openshop.core.event.SystemEventGroup"/>
        <property name="name" />
        <property name="descr" />
    </class>

When I do the schema update I only get a table created with id, name and descr, additionally an SQL exception is thrown because Hibernate trys to execute the following statement: "select from ". (hibernate.show_sql=true).

Any clue, how I can solve this problem?

Thanks for your help.

juergen

bertrand · **Posted:** Tue Feb 17, 2004 7:25 am

Please read the section that deals with one-to-one relationships.

I think the problems comes from the fact that two entities linked by a one-to-one relationship will share the same ID (at least by default - never tried). This, of course, will not work in your case ('cause same table).

Anyway, you could achieve exactly the same behavior (at least in your case) by using a many-to-one from the child to the parent...

michael · **Posted:** Tue Feb 17, 2004 7:28 am

Phew, thats a hard one ... I never saw anyone do this. I think you will have to try to map the relationship property explicitly (eg. map parentId) and use the property-ref attribute in the one-to-one ... probably this might work out.

jdenzel · **Joined:** Mon Jan 05, 2004 10:46 am **Posts:** 13

Thanks for your help.

I will try your solutions.

Best regards,

juergen

jdenzel · **Joined:** Mon Jan 05, 2004 10:46 am **Posts:** 13

Hi,

I have tried your solution proposal, but unfortunatly it does not work. I do not think what I am trying to do so unusual that Hibernate cannot deal with it.

Basically its a linked list which I would like to map to one database table.

I also tried to change my object model, just to test if this could work.
My class looks like this now.

Code:

public final class SystemEventGroup
{
    private long id;
    private long eventKey;
    private Set parentEventGroups = new HashSet();
    private String name;
    private String descr;
...
}

This is the mapping I am using:

Code:

    <class name="wilken.openshop.core.event.SystemEventGroup" table="system_event_group">
        <id name="id" type="long" unsaved-value="-1">
            <column name="id" sql-type="int(11)" not-null="true"/>
            <generator class="identity"/>
        </id>
        <property name="eventKey" column="event_key"/>
        <set name="parentEventGroups" table="system_event_group">
            <key column="parent_key"/>
            <one-to-many class="wilken.openshop.core.event.SystemEventGroup"/>
        </set>                
        <property name="name" />
        <property name="descr" />
    </class>

If I do the schema update I still get this strange sql statement "select from " executed by Hibernate which obviously results in a sql exception.

When trying to store an Object which has a parent the child gets inserted but as soon as the parent should be inserted the following exception is thrown. This will not happen if I use two different classes.

Code:

01:06,053 DEBUG [SessionImpl] opened session
01:06,053 DEBUG [JDBCTransaction] begin
01:06,053 DEBUG [JDBCTransaction] current autocommit status:true
01:06,053 DEBUG [JDBCTransaction] disabling autocommit
01:06,069 DEBUG [SessionImpl] saving [wilken.openshop.core.event.SystemEventGroup#<null>]
01:06,069 DEBUG [EntityPersister] Inserting entity: wilken.openshop.core.event.SystemEventGroup (native id)
01:06,069 DEBUG [BatcherImpl] about to open: 0 open PreparedStatements, 0 open ResultSets
01:06,069 DEBUG [SQL] insert into system_event_group (event_key, name, descr) values (?, ?, ?)
Hibernate: insert into system_event_group (event_key, name, descr) values (?, ?, ?)
01:06,069 DEBUG [BatcherImpl] preparing statement
01:06,069 DEBUG [EntityPersister] Dehydrating entity: [wilken.openshop.core.event.SystemEventGroup#<null>]
01:06,069 DEBUG [LongType] binding '0' to parameter: 1
01:06,069 DEBUG [StringType] binding 'event1' to parameter: 2
01:06,069 DEBUG [StringType] binding null to parameter: 3
01:06,069 DEBUG [BatcherImpl] done closing: 0 open PreparedStatements, 0 open ResultSets
01:06,069 DEBUG [BatcherImpl] closing statement
01:06,069 DEBUG [BatcherImpl] about to open: 0 open PreparedStatements, 0 open ResultSets
01:06,069 DEBUG [SQL] SELECT LAST_INSERT_ID()
Hibernate: SELECT LAST_INSERT_ID()
01:06,069 DEBUG [BatcherImpl] preparing statement
01:06,069 DEBUG [EntityPersister] Natively generated identity: 20
01:06,069 DEBUG [BatcherImpl] done closing: 0 open PreparedStatements, 0 open ResultSets
01:06,069 DEBUG [BatcherImpl] closing statement
01:06,069 DEBUG [JDBCTransaction] commit
01:06,069 DEBUG [SessionImpl] flushing session
01:06,069 DEBUG [SessionImpl] Flushing entities and processing referenced collections
01:06,100 DEBUG [WrapVisitor] Wrapped collection in role: wilken.openshop.core.event.SystemEventGroup.parentEventGroups
01:06,100 DEBUG [SessionImpl] Collection found: [wilken.openshop.core.event.SystemEventGroup.parentEventGroups#20], was: [<unreferenced>]
01:06,100 DEBUG [SessionImpl] Processing unreferenced collections
01:06,100 DEBUG [SessionImpl] Scheduling collection removes/(re)creates/updates
01:06,116 DEBUG [SessionImpl] Flushed: 0 insertions, 0 updates, 0 deletions to 1 objects
01:06,116 DEBUG [SessionImpl] Flushed: 1 (re)creations, 0 updates, 0 removals to 1 collections
01:06,116 DEBUG [Printer] listing entities:
01:06,116 DEBUG [Printer] wilken.openshop.core.event.SystemEventGroup{eventKey=0, descr=null, name=event1, parentEventGroups=[SystemEventGr
01:06,116 DEBUG [SessionImpl] executing flush
01:06,116 DEBUG [BasicCollectionPersister] Inserting collection: [wilken.openshop.core.event.SystemEventGroup.parentEventGroups#20]
01:06,116 DEBUG [BatcherImpl] about to open: 0 open PreparedStatements, 0 open ResultSets
01:06,116 DEBUG [SQL] update system_event_group set parent_key=? where id=?
Hibernate: update system_event_group set parent_key=? where id=?
01:06,116 DEBUG [BatcherImpl] preparing statement
01:06,116 DEBUG [LongType] binding '20' to parameter: 1
01:06,116 DEBUG [Cascades] id unsaved-value: -1
01:06,116 DEBUG [LongType] binding '0' to parameter: 2
01:06,116 DEBUG [BatcherImpl] Adding to batch
01:06,116 DEBUG [BasicCollectionPersister] done inserting collection: 1 rows inserted
01:06,116 DEBUG [BatcherImpl] Executing batch size: 1
01:06,131 DEBUG [BatcherImpl] done closing: 0 open PreparedStatements, 0 open ResultSets
01:06,131 DEBUG [BatcherImpl] closing statement
01:06,131 ERROR [SessionImpl] Could not synchronize database state with session
net.sf.hibernate.HibernateException: Batch update row count wrong: 0
        at net.sf.hibernate.impl.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:65)
        at net.sf.hibernate.impl.BatcherImpl.executeBatch(BatcherImpl.java:118)
        at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2311)
        at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2265)
        at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2187)
        at net.sf.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:61)
        at wilken.openshop.test.TestBean.store(TestBean.java:36)

This is really strange, I do not have a clue why.

Any opinion what can be done to solve this problem?

I would like to avoid to adjust my object model in order for Hibernate to be able to work with it.

Thanks for your help.

Best regards,

juergen

emmanuel · **Posted:** Tue Feb 17, 2004 10:26 am

set batch_size=0 to see a more detailed error.
Check unsaved-value

jdenzel · **Joined:** Mon Jan 05, 2004 10:46 am **Posts:** 13

Hi,

I think I found a solution for my problem.

This is my class:

Code:

public final class SystemEventGroup
{
    private Long id;
    private long eventKey;
    private SystemEventGroup parentEventGroup;
    private String name;
    private String descr;
...
}

This is the mapping which does the job for me:

Code:

    <class name="wilken.openshop.core.event.SystemEventGroup" table="system_event_group">
        <id name="id" type="long" unsaved-value="-1">
            <column name="id" sql-type="int(11)" not-null="true"/>
            <generator class="identity"/>
        </id>
        <property name="eventKey" column="event_key"/>
        <many-to-one name="parentEventGroup" 
            class="wilken.openshop.core.event.SystemEventGroup" 
            column="parent_key" 
            property-ref="eventKey" 
            cascade="all"/>
        <property name="name" />
        <property name="descr" />
    </class>

Thanks for your help.

Best regards,

juergen