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POHow to implement polymorphic JPA entities with generic relations
 

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    <p>I'm trying to use JPA 2.0 to create polymorphic entities with generic relations. There should be two tables, an event table and a notification table. Inside those table are concrete entities that are related to one another, like so:</p> <pre><code>Event &lt;---------- Notification&lt;X extends Event&gt; | | LoginEvent &lt;------ LoginNotification extends Notification&lt;LoginEvent&gt; </code></pre> <p>Logically this should be possible in hibernate, as it is possible in SQL:</p> <pre><code>+----------+ +----------+ | Event | | Notif | +----------+ +----------+ | | | Id | | Id | &lt;- | Evt_id | | Type | &lt;- | Type | | ... | | ... | +----------+ +----------+ </code></pre> <p>This is what I have:</p> <pre><code>@Entity @Inheritance public abstract class Event{ ... } @Entity public class LoginEvent extends Event{ ... } @Entity @Inheritance public abstract class Notification&lt;X extends Event&gt;{ @ManyToOne(optional=false, targetEntity=Event.class) @JoinColumn private X event; ... } @Entity public class LoginNotification extends Notification&lt;LoginEvent&gt;{ ... } </code></pre> <p>Using this code, I can persist and fetch any Event, Notification, LoginEvent, or NotificationEvent, but it falls down when I try to use the <code>LoginNotification_.event</code> relation in my JPA 2.0 metamodel queries. <a href="http://opensource.atlassian.com/projects/hibernate/browse/METAGEN-29" rel="noreferrer">This issue</a> explains something similar.</p> <pre><code>public static volatile SingularAttribute&lt;NotificationEntity, EventEntity&gt; event; </code></pre> <p>When I try to do a join in a criteria query, I get an error:</p> <pre><code>EntityManager em = getEntityManager(); CriteriaBuilder cb = em.getCriteriaBuilder(); CriteriaQuery&lt;LoginNotification&gt; query = cb.createQuery(LoginNotification.class); Root&lt;LoginNotification&gt; root = query.from(LoginNotification.class); // This line complains: Type mismatch: cannot convert from // Join&lt;LoginNotification,Event&gt; to Join&lt;LoginNotification,LoginEvent&gt; Join&lt;LoginNotification, LoginEvent&gt; join = root.join(LoginNotification_.event, JoinType.INNER); </code></pre> <p>I can get around this error, by adding a new <code>SingularAttribute</code> to the <code>LoginNotification_</code> metamodel, but this fails in execution:</p> <pre><code>public abstract class LoginNotification_ extends Notification_ { // Adding this Removes Type mismatch error, but causes run-time error public static volatile SingularAttribute&lt;LoginNotification, LoginEvent&gt; event; ... } </code></pre> <p>According to some posts, generic relations won't work (<a href="http://stackoverflow.com/questions/2808251/how-to-handle-jpa-annotations-for-a-pointer-to-a-generic-interface">How to handle JPA annotations for a pointer to a generic interface</a>), but by using a <code>@ManyToOne(optional=false, targetEntity=Event.class)</code> annotation, we can get them to behave. Unfortunately, the generics seem to break the JPA criteria query.</p> <p>Are there any suggestions on how I can perform this lookup? I can use <code>LoginNotification.getEvent()</code> in my code, but I cannot use <code>LoginNotification_.event</code> in my JPA metamodel joins. What's the alternative to using generics to accomplish this?</p> <p>@Pascal Thivent - Can you answer this?</p>
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