Hi,

In my legacy DB, we often have a parent table which has a PK from a sequnce. Then parent has one or more child tables which uses parent PK as not null FK to form composite key for child tables.

From application side, Presentation tier would like to build up Parent/Child object graph and pass to server DAO in one request. and DAO should ideally persist the graph in one shot. Please note the Child class is entired "contained" within the Parent class, i.e. Child class's life-cycle is bound by the Parent class and any access to Child class must go thru the Parent class.

So far I have experimented with bidirectional and unidirectional one-to-many mapping without success. The problem is that the when Presentation tier constuct the child composite id class. It does not know what's the parent id (not obtained until DAO ask DB), so it leave it as NULL. When DAO persists the Child class with NULL parent id in its composite key, DB throws constraint violation exception.

The following is my bidrectional mapping and sample Java code. I wonder what's the best practice people have found for this common problem - I prefer changing the mapping file while still leave the Presentation tier to build the object graph:

// DB DDL

CREATE TABLE Parent (
parentId INTEGER NOT NULL
);

ALTER TABLE Parent
ADD ( PRIMARY KEY (parentId) ) ;



CREATE TABLE Child (
parentId INTEGER NOT NULL,
childNumber INTEGER NOT NULL
);

ALTER TABLE Child
ADD ( PRIMARY KEY (parentId, childNumber) ) ;


// hibernate mapping

<class name="Parent" table="PARENT">
<id name="parentid" type="long">
<column name="PARENTID" precision="22" scale="0" />
<generator class="sequence">
<param name="sequence">seq_parentId</param>
</generator>
</id>
<set name="childs" inverse="true" cascade="all-delete-orphan">
<key>
<column name="PARENTID" precision="22" scale="0" not-null="true" />
</key>
<one-to-many class="Child" />
</set>
</class>

<class name="Child" table="CHILD">
<composite-id name="id" class="ChildId">
<key-property name="parentid" type="long">
<column name="PARENTID" precision="22" scale="0" />
</key-property>
<key-property name="childnumber" type="long">
<column name="CHILDNUMBER" precision="22" scale="0" />
</key-property>
</composite-id>
<many-to-one name="parent" class="Parent" update="false" insert="false" fetch="select">
<column name="PARENTID" precision="22" scale="0" not-null="true" />
</many-to-one>
</class>


// simplified Java code:

Parent p = new Parent();
Child c = new Child();
c.setParent(p);
ChildId cid = new ChildId();
cid.setChildnumber(101);
//cid.setParentid(1); // this is unreasoable to Pres
c.setId(cid);
p.getChilds().add(c);

// DAO tier
Session ssn = sessionFactory.openSession();
Transaction tx = ssn.beginTransaction();
Serializable s;
s = ssn.save(p);

tx.commit();
ssn.close();

you need to use subclass in your child class.

why subclass - I think that is only used for inheritance. Parent and Child class are not in any inheritance (or interface/implementation ) relationship

I was taking parent and child to mean inheritence. You just mean to say that one class has another class as a property?

yes. child is contained by parent as a property (collection). child does not inherit from parent. e.g. Address is contained by Person.

Ok.

Either the child or the parent has to allow for a null identifier reference. You then presist that one first. then Persist the other. Finall go back and update the first which will give it the correct reference to the second.

Hi chuckzhg,
Look at post http://forum.hibernate.org/viewtopic.ph ... 23#2304716 , I think it can help, if you don't have to handle with database errors (that is the problem that I am trying to solve...)
If you find any correction to this example, please tell me.

Thx for the reply, panisson. My Person/Address example is somewhat different from yours

If the Child class has its on ID column based on DB sequence, Hibernate has no problem in creating the object graph in one go using regular bidrectional one-to-many mapping.

But my legacy DB's Child class has a composite key:



and my mapping is:


But when I run the following code:



I got exception on NULL ChildId.parentId:



How can Hibernate handle this common use case gracefully?

Maybe the best approach is to include the many-to-one in the composite-id:



and include the Parent in the ChildId:

Try using a foreign generator strategy to populate the child's FK value

-- snippet from Hibernate reference (5.1.11)

Thanks a lot panisson.

So it seems for hibernate to handle composite-id composed of foreign key, the only working solution is using <key-many-to-one> and have the ChildId class hold a reference to the Parent class

Discussed with Gavin King in JavaOne today about this issue. He does not recomend <key-many-to-one> (I did not ask why). He recommend Parent id uses generator="assigned" and it's up to application to get the id (from DB sequence or whereever) and set into both parent and child keys. I am wondering do I really want my pres tier to get next value from DB sequence? Opinions, anyone?