parent and only child relationship

mvanhorn · **Joined:** Mon May 10, 2004 6:53 pm **Posts:** 11

What is the best way to model a parent-child relationship where there can only be one child per parent? Additionally, the parent and child are the same class.

I would rather avoid using a collection to hold the children if possible, and it would be nice to have a bidrectional relationship, although if that is difficult, then I would have the parent control things.

Can anyone give me a pointer, (I am comfortable with hibernate, but not sure of all the best practices)?

christian · **Posted:** Mon May 10, 2004 7:10 pm

If the child is not an entity with shared references, use simple <component> mapping with a <parent> element for bidirectionality.

mvanhorn · **Joined:** Mon May 10, 2004 6:53 pm **Posts:** 11

Since the child is the same class as the parent, I am guessing it would have to be an entity - even though the child instance is dependent on the parent instance.

This is kind of confusing to me - would it look like this?

table:FOO
columns:
pkid int,
bar varchar(100),
parent_id int,
child_id int

Code:

public class Foo {
    String bar;
    Foo myChild;
    Foo myParent;
}

<class name="Foo" table="FOO">
    <id name="id" column="pkid" type="int">
        <generator class="native"/>
    </id>               
    <property name="bar"/>
    <component 
        name="myChild"          
        class="eg.Foo" > 
        <property name="bar" column="bar"/>
        <parent name="myParent"/>
    </component>
</class>

How does the component get mapped to the child_id/parent_id columns ?

christian · **Posted:** Tue May 11, 2004 5:30 am

Hm, yes, that is not what you want. Your object model doesn't make much sense to me, are you saying that you have a linked list? A.getA().getA().getA()... and so on? Why would you need that?

mvanhorn · **Joined:** Mon May 10, 2004 6:53 pm **Posts:** 11

I am trying to model part of an XML schema which is constructed like this.

THis particular part is for a hierarchy of organization names. For example a person works at Acme RocketSkates, Roadrunner Pursuit division, in the R&D section. The XML would look like this:

Code:

<PositionOrgName>
    <OrgType>Company</OrgType>
    <OrganizationName>Acme RocketSkates</OrganizationName>
    <PositionOrgName>
        <OrgType>Division</OrgType>
        <OrganizationName>Roadrunner Pursuit</OrganizationName>
        <PositionOrgName>
            <OrgType>Section</OrgType>
            <OrganizationName>R and D</OrganizationName>
        </PositionOrgName>
    </PositionOrgName>
</PositionOrgName>

christian · **Posted:** Tue May 11, 2004 6:00 am

I think you have the wrong approach. Relational models are normalized, removing redundancy, while hierarchical models are not (and therefore, they are useless for data management). There is no easy answer to your problem, you will have to read up on relational data modeling.

mvanhorn · **Joined:** Mon May 10, 2004 6:53 pm **Posts:** 11

Yeah, I think I knew where this was headed. Most of this schema translates well to a relational model, there are only a few areas where it doesn't. So I guess I will leave out support for this part of it for now, until I can come up with something decent.

Unfortunately, although most of this model deals with things that can be modeled in a rdb, there are areas like this where the aim is to have a flexible structure (accomodating a hierarchy of unknown depth and org types) and this doesn't fit well into a relational model.

It is too bad that none of the free xml databases seem able to meet the other requirements, or I might have gone that route. I guess my job will be to figure out some kind of balance between supporting this spec, and having a performant app.