public final class Key implements Serializable {

	static final long serialVersionUID = -448150158203091507L;
	public String targetClassName;
	public String id;
	
	public Key() {	}
	
	public Key(String str) {
		init(str);
	}
	
	private void init(String str) {
		if(StringUtils.isEmpty(str)) {
			targetClassName = "";
			id = "";
			return;
		}
		String[] parts = str.split("\\(");
		parts[1] = parts[1].replaceAll("\\)", " ");
		parts[1] = parts[1].replace("\"", " ");
		parts[1] = parts[1].trim();
		this.targetClassName = parts[0];
		this.id = parts[1];
	}
	
	public void complete() {
		init(id);
	}

	public Key(String classCollectionName, String id) {
		if (StringUtils.isEmpty(classCollectionName)) {
			throw new IllegalArgumentException("No collection/class name specified.");
		}
		if (id == null) {
			throw new IllegalArgumentException("ID cannot be null");
		}
		targetClassName = getTargetClassName(classCollectionName);
		this.id = id;
	}

	public int hashCode() {
		int prime = 31;
		int result = 1;
		if(!isComplete()) init(id);
		result = prime * result + id.hashCode();
		result = prime * result + targetClassName.hashCode();
		return result;
	}

	public boolean equals(Object object) {
		if(!isComplete()) init(id);
		if (object instanceof Key) {
			Key key = (Key) object;
			if (this == key)
				return true;
			return targetClassName.equals(key.targetClassName) && Objects.equals(id, key.id);
		} else {
			return false;
		}
	}

	public String toString() {
		if(!isComplete()) init(id);
		StringBuilder buffer = new StringBuilder();
		int index = targetClassName.lastIndexOf('.');
		buffer.append(index < 0 ? targetClassName : targetClassName.substring(++index));
		buffer.append("(");
		if (id != null) {
			buffer.append("\"").append(id)
					.append("\"").toString());
		} else {
			buffer.append("no-id-yet");
		}
		buffer.append(")");
		return buffer.toString();
	}

	public boolean isComplete() {
		return targetClassName != null && id != null;
	}

}

@PersistenceCapable(detachable="true", objectIdClass=Key.class)
@Inheritance(strategy=InheritanceStrategy.COMPLETE_TABLE)
public abstract class Entity {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.UNSPECIFIED, column="_id")
    public String id;
...
}

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
        http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd" version="2.0">

    <!-- JOSAdmin "unit" -->
    <persistence-unit name="ourdatastore">
        <class>mx.ourdomain.Entiy</class>
        <class>mx.ourdomain.Ticket</class>
        <exclude-unlisted-classes/>

    </persistence-unit>
</persistence>

<?xml version="1.0"?>
<!DOCTYPE orm SYSTEM "file:/javax/jdo/orm.dtd">
<orm>
    <package name="mx.ourdomain" >
        <class name="Entity" table="Entity">
            <field name="id" primary-key="true" >
                <column name="_id" length="100" />
            </field >
        </class>

        <class name="Ticket" table="Ticket">
            <primary-key >
                <column name="_id" target="_id" />
            </primary-key>
        </class>
     </package>
</orm>