Question

Final goal:
Have a few java objects sharing the same base class persisted into a database while each one having its own self-contained table with all own/inherited objects, and a simple auto-generated by the database id.

Very simple requirement. Impossible (?) with Hibernate!

What I have so far (using MySQL, Hibernate XML mapping):

map.hbm.xml

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping default-access="field"
    default-lazy="false">

    <class name="xyz.url.hibernate.EntityBase" abstract="true">
        <id name="m_id" column="id">
            <generator class="identity" />
        </id>

        <version name="m_version" column="version" type="long" />

        <union-subclass name="xyz.url.hibernate.User" table="my_entity">
            <property name="name" column="name" type="string" />
        </union-subclass>
    </class>

</hibernate-mapping>

EntityBase.java

public abstract class EntityBase {
    private final long  m_id;
    private final long  m_version;

    public EntityBase() {
        this.m_id = 0;
        this.m_version = 0;
    }

    public long get_id() {
        return this.m_id;
    }

    public long get_version() {
        return this.m_version;
    }
}

User.java

public class User extends EntityBase {
    public String   name;
}

The above does not work unless you change the generator class to increment.
Currently this is the given error:

org.hibernate.MappingException: Cannot use identity column key generation with <union-subclass> mapping for: xyz.url.hibernate.User

Well, why does Hibernate ASSUMES I want a unique ID in a program-wide scope (I’ve read about some JPA requirement)… what a crap!

Anyway, I insist of having a simple table (per object) that aggregates all the object’s (User in this case) properties, and deny using discriminators (again what a crap..) which just complicate the final SQL queries and hit performance.

The only solutions I see here:

1. Manually map all properties in a one block inside the XML.

2. Map all properties while “importing” some <propert> items from an external file, thus achieve inheritance (reusage of properties). Possible? How to do?!?

3. Explore annotations further which as far as I’ve seen they don’t support that simple inheritance requirement.

4. Dump Hibernate and go with another ORM solution.

Please don’t link to the docs – I gave up on that one after reading them a few times!
An example of property import (from external file) would be great.

Thanks and god bless!

Answer 1

First of all you need to decide whether your inheritance relationship should be mapped to the database (to allow polymorphic queries such as from EntityBase, polymorphic realtionships, etc) or not.

As far as I understand in your case it shouldn’t be mapped, therefore it doesn’t make sense to use inheritance mapping options such as <union-subclass> at all. Now you have the following options:

2. Hibernate doesn’t have special support for reuse of XML mappings, but its documentation suggests to use XML entities in this case, see, for example, 10.1.6. Table per concrete class using implicit polymorphism.

3. Annotations certainly support this requirement in form of @MappedSuperclass annotation.
This annotation can be used to mark a class that is not mapped to the database itself, but any mapping annotations defined on its properties take effect for its mapped subclasses, so that you don’t need to repeat them.