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Question

How do you map a collection of base type objects in a derived type of the same inheritance tree?

Answer 1

+1 A:

There are two issues here, both are with the mapping file.

The Component is a primitive, it cannot be further divided and thus has no subcomponents. The Part is a composite and thus can have multiple subcomponents that are either Components, Parts, or some combination there of. Therefore, a Component is the base class and Part is a derived class which adds a collection of Components. In this relationship, the Component objects are what is being collected and the Part object is what is holding that collection.

Issue #1 - The set key

The set called "Subcomponents" is properly placed in the Part class, however the key column incorrectly points to the ComponentID when it should point to the PartID. Per the Hibernate Reference Documentation Section 6.2.1. Collection foreign keys:

Collection instances are distinguished in the database by the foreign key of the entity that owns the collection.

So here, since the Part is what owns the collection, the PartID should be used to key it.

Issue #2 - Inverse

From Hibernate - Inside explanation of inverse=true

Inverse defines which side is responsible for the association maintenance. The side having inverse="false" (default value) has this responsibility and will create the appropriate SQL query. Changes made to the association on the side of the inverse="true" are not persisted in the DB.

So here, since this isn't a bi-directional association, the Part is responsible for the collection and should be marked inverse="false" (or leave out this attribute entirely since the default is false). If you don't mark it as false and instead mark it as true, you'll end up with an empty PartID value in the Component table for Subcomponents and possibly duplicate Components. In otherwords, it will persist all of the objects in the collection, but it won't persist the relationship information that tells Hibernate which components belong in which Part's collection.

Code

The correct Hibernate Mapping

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
                        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
                        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"&gt;

<hibernate-mapping package="manufacturing">
    <class name="Component" table="Component">
        <id name="ID" column="ComponentID">
            <generator class="native" />
        </id>
        <property name="name" column="Name" />
        <property name="price" column="Price" />        

        <joined-subclass name="Part" table="Part">
            <key column="PartID" />
            <!-- A part has has [0..n] sub-components -->
            <set name="Subcomponents" lazy="true" cascade="all" inverse="false">
                <key column="PartID" />
                <one-to-many class="Component" />
            </set>

        </joined-subclass>
    </class>
</hibernate-mapping>

The resulting schema:

create table Component (ComponentID bigint generated by default as identity (start with 1), Name varchar(255), Price float, PartID bigint, primary key (ComponentID));
create table Part (PartID bigint not null, primary key (PartID));
alter table Component add constraint FK24013CDD89368311 foreign key (PartID) references Part;
alter table Part add constraint FK25D813296E407B foreign key (PartID) references Component;

Burly 2010-07-23 13:45:26

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How do you map a collection of base type objects in a derived type of the same inheritance tree?

Background

Assumptions

Problem

Question

Code

Issue #1 - The set key

Issue #2 - Inverse

Code

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